### 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.
```
* Add timeout example
* Add pb2 file to example
* Remove .proto file
* Add keep_alive example
* Add refelction client
* fixes
* Add example for health_check
* Changes based on comments
* Fix pylint
* Update third_party/protobuf
* run tools/distrib/python/make_grpcio_tools.py
* regenerate protos for ruby, php
* update build_handwritten.yaml
* regenerate projects
* Build - Use :well_known_type_protos instead of :well_known_protos
* Fix target
* Update upb
* Update Python for Protobuf 4.21 (#140
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)
* Update protobuf dependency on grpcio-tools
* Off by one
* Drop python 3.6 support
* Try upgrading pip
* And in the other script
* Try to figure out if we're compatible with abi3
* See what we've already got installed
* Update the requirements.txt file I didn't know existed
* And here too
* See what's installed
* Let's try that again
* Remove
* Try to confirm version
* Let me see the generated code
* Fix non-Bazel test runner
* Work for all test directories
* Regenerate example protos
* Clean up
* Generate .pyi files
* Fix type checking and linting
* Exclude pyi files from isort
* Upgrade to 3.21.4
* Update iwyu to get around messy protobuf IWYU rules
Co-authored-by: Richard Belleville <gnossen@gmail.com>
Xuan Wang
Yash Tibrewal
Richard Belleville
Richard Belleville
Nathaniel Manista
yang-g