Use metadata types in the service context

Replace the signature to allow methods to use the metadata object.
Internally, they'll still wrap the data in a tuple, but the interface
makes it clear that the ``aio.Metadata()`` object is supported.

Remove the ``tuple(<metadata>)`` conversions done in the tests.

src/python/grpcio/grpc/_cython/_cygrpc/aio/server.pyx.pxi

@@ -143,7 +143,7 @@ cdef class _ServicerContext:
                             self._loop)
         self._rpc_state.metadata_sent = True
 
-    async def send_initial_metadata(self, tuple metadata):
+    async def send_initial_metadata(self, object metadata):
         self._rpc_state.raise_for_termination()
 
         if self._rpc_state.metadata_sent:
@@ -151,7 +151,7 @@ cdef class _ServicerContext:
         else:
             await _send_initial_metadata(
                 self._rpc_state,
-                _augment_metadata(metadata, self._rpc_state.compression_algorithm),
+                _augment_metadata(tuple(metadata), self._rpc_state.compression_algorithm),
                 _EMPTY_FLAG,
                 self._loop
             )
@@ -192,8 +192,8 @@ cdef class _ServicerContext:
     async def abort_with_status(self, object status):
         await self.abort(status.code, status.details, status.trailing_metadata)
 
-    def set_trailing_metadata(self, tuple metadata):
+    def set_trailing_metadata(self, object metadata):
-        self._rpc_state.trailing_metadata = metadata
+        self._rpc_state.trailing_metadata = tuple(metadata)
 
     def invocation_metadata(self):
         return self._rpc_state.invocation_metadata()
@@ -233,13 +233,13 @@ cdef class _SyncServicerContext:
         # Abort should raise an AbortError
         future.exception()
 
-    def send_initial_metadata(self, tuple metadata):
+    def send_initial_metadata(self, object metadata):
         future = asyncio.run_coroutine_threadsafe(
             self._context.send_initial_metadata(metadata),
             self._loop)
         future.result()
 
-    def set_trailing_metadata(self, tuple metadata):
+    def set_trailing_metadata(self, object metadata):
         self._context.set_trailing_metadata(metadata)
 
     def invocation_metadata(self):

src/python/grpcio_tests/tests_aio/unit/metadata_test.py

@@ -104,13 +104,13 @@ class _TestGenericHandlerForMethods(grpc.GenericRpcHandler):
     async def _test_server_to_client(request, context):
         assert _REQUEST == request
         await context.send_initial_metadata(
-            tuple(_INITIAL_METADATA_FROM_SERVER_TO_CLIENT))
+            _INITIAL_METADATA_FROM_SERVER_TO_CLIENT)
         return _RESPONSE
 
     @staticmethod
     async def _test_trailing_metadata(request, context):
         assert _REQUEST == request
-        context.set_trailing_metadata(tuple(_TRAILING_METADATA))
+        context.set_trailing_metadata(_TRAILING_METADATA)
         return _RESPONSE
 
     @staticmethod
@@ -119,21 +119,21 @@ class _TestGenericHandlerForMethods(grpc.GenericRpcHandler):
         assert _common.seen_metadata(_INITIAL_METADATA_FROM_CLIENT_TO_SERVER,
                                      context.invocation_metadata())
         await context.send_initial_metadata(
-            tuple(_INITIAL_METADATA_FROM_SERVER_TO_CLIENT))
+            _INITIAL_METADATA_FROM_SERVER_TO_CLIENT)
         yield _RESPONSE
-        context.set_trailing_metadata(tuple(_TRAILING_METADATA))
+        context.set_trailing_metadata(_TRAILING_METADATA)
 
     @staticmethod
     async def _test_stream_unary(request_iterator, context):
         assert _common.seen_metadata(_INITIAL_METADATA_FROM_CLIENT_TO_SERVER,
                                      context.invocation_metadata())
         await context.send_initial_metadata(
-            tuple(_INITIAL_METADATA_FROM_SERVER_TO_CLIENT))
+            _INITIAL_METADATA_FROM_SERVER_TO_CLIENT)
 
         async for request in request_iterator:
             assert _REQUEST == request
 
-        context.set_trailing_metadata(tuple(_TRAILING_METADATA))
+        context.set_trailing_metadata(_TRAILING_METADATA)
         return _RESPONSE
 
     @staticmethod
@@ -141,13 +141,13 @@ class _TestGenericHandlerForMethods(grpc.GenericRpcHandler):
         assert _common.seen_metadata(_INITIAL_METADATA_FROM_CLIENT_TO_SERVER,
                                      context.invocation_metadata())
         await context.send_initial_metadata(
-            tuple(_INITIAL_METADATA_FROM_SERVER_TO_CLIENT))
+            _INITIAL_METADATA_FROM_SERVER_TO_CLIENT)
 
         async for request in request_iterator:
             assert _REQUEST == request
 
         yield _RESPONSE
-        context.set_trailing_metadata(tuple(_TRAILING_METADATA))
+        context.set_trailing_metadata(_TRAILING_METADATA)
 
     def service(self, handler_call_details):
         return self._routing_table.get(handler_call_details.method)