[Python fix-it][Aysnc Example] Adding examples for aysnc interceptor context propagaton (#32840)

### Description

Fix https://github.com/grpc/grpc/issues/24470.

Adding one example which demonstrate the following use cases:

* Generate RPC ID on client side and propagate to server.
* Context propagation from client to server.
* Context propagation between different server interceptors and the
server handler.

## Use:

1. Start server: `python3 -m async_greeter_server_with_interceptor`
2. Start client: `python3 -m async_greeter_client`

### Expected Logs:

* On client side:

```
Sending request with rpc id: 73bb98beff10c2dd7b9f2252a1e2039e
Greeter client received: Hello, you!
```

* On server side:

```
INFO:root:Starting server on [::]:50051
INFO:root:Interceptor1 called with rpc_id: default
INFO:root:Interceptor2 called with rpc_id: Interceptor1-default
INFO:root:Handle rpc with id Interceptor2-Interceptor1-73bb98beff10c2dd7b9f2252a1e2039e in server handler.
```

doc/python/sphinx/grpc.rst

@@ -112,7 +112,7 @@ gRPC Exceptions
 .. autoexception:: RpcError
 
 
-Shared Context
+RPC Context
 ^^^^^^^^^^^^^^
 
 .. autoclass:: RpcContext

doc/python/sphinx/grpc_asyncio.rst

@@ -82,7 +82,7 @@ gRPC Metadata
 .. autoclass:: Metadata
 
 
-Shared Context
+RPC Context
 ^^^^^^^^^^^^^^^^^^^^
 
 .. autoclass:: RpcContext

examples/python/interceptors/async/README.md

@@ -0,0 +1,43 @@
+# gRPC Python Async Interceptor Example
+
+This example demonstrate the usage of Async interceptors and context propagation using [contextvars](https://docs.python.org/3/library/contextvars.html#module-contextvars).
+
+## When to use contextvars
+
+`Contextvars` can be used to propagate context in a same thread or coroutine, some example usage include:
+
+1. Propagate from interceptor to another interceptor.
+2. Propagate from interceptor to the server handler.
+3. Propagate from client to server.
+
+## How does this example works
+
+This example have the following steps:
+1. Generate RPC ID on client side and propagate to server using `metadata`.
+    * `contextvars` can be used here if client and server is running in a same coroutine (or same thead for Sync).
+2. Server interceptor1 intercept the request, it checks `rpc_id_var` and decorate it with it's tag `Interceptor1`.
+3. Server interceptor2 intercept the request, it checks `rpc_id_var` and decorate it with it's tag `Interceptor2`.
+4. Server handler receives the request with `rpc_id_var` decorated by both interceptor1 and interceptor2.
+
+## How to run this example
+
+1. Start server: `python3 -m async_greeter_server_with_interceptor`
+2. Start client: `python3 -m async_greeter_client`
+
+### Expected outcome
+
+* On client side, you should see logs similar to:
+
+```
+Sending request with rpc id: 59ac966558b3d7d11a06bd45f1a0f89d
+Greeter client received: Hello, you!
+```
+
+* On server side, you should see logs similar to:
+
+```
+INFO:root:Starting server on [::]:50051
+INFO:root:Interceptor1 called with rpc_id: default
+INFO:root:Interceptor2 called with rpc_id: Interceptor1-59ac966558b3d7d11a06bd45f1a0f89d
+INFO:root:Handle rpc with id Interceptor2-Interceptor1-59ac966558b3d7d11a06bd45f1a0f89d in server handler.
+```

examples/python/interceptors/async/async_greeter_client.py

@@ -0,0 +1,41 @@
+# Copyright 2023 gRPC authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+"""The Python AsyncIO implementation of the GRPC helloworld.Greeter client."""
+
+import asyncio
+import contextvars
+import logging
+import random
+
+import grpc
+import helloworld_pb2
+import helloworld_pb2_grpc
+
+test_var = contextvars.ContextVar('test', default='test')
+
+
+async def run() -> None:
+    async with grpc.aio.insecure_channel('localhost:50051') as channel:
+        stub = helloworld_pb2_grpc.GreeterStub(channel)
+        rpc_id = '{:032x}'.format(random.getrandbits(128))
+        metadata = grpc.aio.Metadata(('client-rpc-id', rpc_id),)
+        print(f"Sending request with rpc id: {rpc_id}")
+        response = await stub.SayHello(helloworld_pb2.HelloRequest(name='you'),
+                                       metadata=metadata)
+    print("Greeter client received: " + response.message)
+
+
+if __name__ == '__main__':
+    logging.basicConfig()
+    asyncio.run(run())

examples/python/interceptors/async/async_greeter_server_with_interceptor.py

@@ -0,0 +1,83 @@
+# Copyright 2023 gRPC authors.
+#
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+"""The Python AsyncIO implementation of the GRPC helloworld.Greeter server."""
+
+import asyncio
+import contextvars
+import logging
+from typing import Awaitable, Callable, Optional
+
+import grpc
+import helloworld_pb2
+import helloworld_pb2_grpc
+
+rpc_id_var = contextvars.ContextVar('rpc_id', default='default')
+
+
+class RPCIdInterceptor(grpc.aio.ServerInterceptor):
+
+    def __init__(self, tag: str, rpc_id: Optional[str] = None) -> None:
+        self.tag = tag
+        self.rpc_id = rpc_id
+
+    async def intercept_service(
+            self, continuation: Callable[[grpc.HandlerCallDetails],
+                                         Awaitable[grpc.RpcMethodHandler]],
+            handler_call_details: grpc.HandlerCallDetails
+    ) -> grpc.RpcMethodHandler:
+        """
+        This interceptor prepends its tag to the rpc_id.
+        If two of these interceptors are chained together, the resulting rpc_id
+        will be something like this: Interceptor2-Interceptor1-RPC_ID.
+        """
+        logging.info("%s called with rpc_id: %s", self.tag, rpc_id_var.get())
+        if rpc_id_var.get() == 'default':
+            _metadata = dict(handler_call_details.invocation_metadata)
+            rpc_id_var.set(self.decorate(_metadata['client-rpc-id']))
+        else:
+            rpc_id_var.set(self.decorate(rpc_id_var.get()))
+        return await continuation(handler_call_details)
+
+    def decorate(self, rpc_id: str):
+        return f"{self.tag}-{rpc_id}"
+
+
+class Greeter(helloworld_pb2_grpc.GreeterServicer):
+
+    async def SayHello(
+            self, request: helloworld_pb2.HelloRequest,
+            context: grpc.aio.ServicerContext) -> helloworld_pb2.HelloReply:
+        logging.info("Handle rpc with id %s in server handler.",
+                     rpc_id_var.get())
+        return helloworld_pb2.HelloReply(message='Hello, %s!' % request.name)
+
+
+async def serve() -> None:
+    interceptors = [
+        RPCIdInterceptor('Interceptor1'),
+        RPCIdInterceptor('Interceptor2')
+    ]
+
+    server = grpc.aio.server(interceptors=interceptors)
+    helloworld_pb2_grpc.add_GreeterServicer_to_server(Greeter(), server)
+    listen_addr = '[::]:50051'
+    server.add_insecure_port(listen_addr)
+    logging.info("Starting server on %s", listen_addr)
+    await server.start()
+    await server.wait_for_termination()
+
+
+if __name__ == '__main__':
+    logging.basicConfig(level=logging.INFO)
+    asyncio.run(serve())

examples/python/interceptors/async/helloworld_pb2.py

@@ -0,0 +1,30 @@
+# -*- coding: utf-8 -*-
+# Generated by the protocol buffer compiler.  DO NOT EDIT!
+# source: helloworld.proto
+"""Generated protocol buffer code."""
+from google.protobuf.internal import builder as _builder
+from google.protobuf import descriptor as _descriptor
+from google.protobuf import descriptor_pool as _descriptor_pool
+from google.protobuf import symbol_database as _symbol_database
+# @@protoc_insertion_point(imports)
+
+_sym_db = _symbol_database.Default()
+
+
+
+
+DESCRIPTOR = _descriptor_pool.Default().AddSerializedFile(b'\n\x10helloworld.proto\x12\nhelloworld\"\x1c\n\x0cHelloRequest\x12\x0c\n\x04name\x18\x01 \x01(\t\"\x1d\n\nHelloReply\x12\x0f\n\x07message\x18\x01 \x01(\t2I\n\x07Greeter\x12>\n\x08SayHello\x12\x18.helloworld.HelloRequest\x1a\x16.helloworld.HelloReply\"\x00\x42\x36\n\x1bio.grpc.examples.helloworldB\x0fHelloWorldProtoP\x01\xa2\x02\x03HLWb\x06proto3')
+
+_builder.BuildMessageAndEnumDescriptors(DESCRIPTOR, globals())
+_builder.BuildTopDescriptorsAndMessages(DESCRIPTOR, 'helloworld_pb2', globals())
+if _descriptor._USE_C_DESCRIPTORS == False:
+
+  DESCRIPTOR._options = None
+  DESCRIPTOR._serialized_options = b'\n\033io.grpc.examples.helloworldB\017HelloWorldProtoP\001\242\002\003HLW'
+  _HELLOREQUEST._serialized_start=32
+  _HELLOREQUEST._serialized_end=60
+  _HELLOREPLY._serialized_start=62
+  _HELLOREPLY._serialized_end=91
+  _GREETER._serialized_start=93
+  _GREETER._serialized_end=166
+# @@protoc_insertion_point(module_scope)

examples/python/interceptors/async/helloworld_pb2.pyi

@@ -0,0 +1,17 @@
+from google.protobuf import descriptor as _descriptor
+from google.protobuf import message as _message
+from typing import ClassVar as _ClassVar, Optional as _Optional
+
+DESCRIPTOR: _descriptor.FileDescriptor
+
+class HelloReply(_message.Message):
+    __slots__ = ["message"]
+    MESSAGE_FIELD_NUMBER: _ClassVar[int]
+    message: str
+    def __init__(self, message: _Optional[str] = ...) -> None: ...
+
+class HelloRequest(_message.Message):
+    __slots__ = ["name"]
+    NAME_FIELD_NUMBER: _ClassVar[int]
+    name: str
+    def __init__(self, name: _Optional[str] = ...) -> None: ...

examples/python/interceptors/async/helloworld_pb2_grpc.py

@@ -0,0 +1,70 @@
+# Generated by the gRPC Python protocol compiler plugin. DO NOT EDIT!
+"""Client and server classes corresponding to protobuf-defined services."""
+import grpc
+
+import helloworld_pb2 as helloworld__pb2
+
+
+class GreeterStub(object):
+    """The greeting service definition.
+    """
+
+    def __init__(self, channel):
+        """Constructor.
+
+        Args:
+            channel: A grpc.Channel.
+        """
+        self.SayHello = channel.unary_unary(
+                '/helloworld.Greeter/SayHello',
+                request_serializer=helloworld__pb2.HelloRequest.SerializeToString,
+                response_deserializer=helloworld__pb2.HelloReply.FromString,
+                )
+
+
+class GreeterServicer(object):
+    """The greeting service definition.
+    """
+
+    def SayHello(self, request, context):
+        """Sends a greeting
+        """
+        context.set_code(grpc.StatusCode.UNIMPLEMENTED)
+        context.set_details('Method not implemented!')
+        raise NotImplementedError('Method not implemented!')
+
+
+def add_GreeterServicer_to_server(servicer, server):
+    rpc_method_handlers = {
+            'SayHello': grpc.unary_unary_rpc_method_handler(
+                    servicer.SayHello,
+                    request_deserializer=helloworld__pb2.HelloRequest.FromString,
+                    response_serializer=helloworld__pb2.HelloReply.SerializeToString,
+            ),
+    }
+    generic_handler = grpc.method_handlers_generic_handler(
+            'helloworld.Greeter', rpc_method_handlers)
+    server.add_generic_rpc_handlers((generic_handler,))
+
+
+ # This class is part of an EXPERIMENTAL API.
+class Greeter(object):
+    """The greeting service definition.
+    """
+
+    @staticmethod
+    def SayHello(request,
+            target,
+            options=(),
+            channel_credentials=None,
+            call_credentials=None,
+            insecure=False,
+            compression=None,
+            wait_for_ready=None,
+            timeout=None,
+            metadata=None):
+        return grpc.experimental.unary_unary(request, target, '/helloworld.Greeter/SayHello',
+            helloworld__pb2.HelloRequest.SerializeToString,
+            helloworld__pb2.HelloReply.FromString,
+            options, channel_credentials,
+            insecure, call_credentials, compression, wait_for_ready, timeout, metadata)