Implement Python CSM observability.
Design: [go/grpc-python-opentelemetry](http://goto.google.com/grpc-python-opentelemetry)
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Closes#36557
PiperOrigin-RevId: 639073741
Add registered method change to sync stack server.
## Note
* We're keeping both generic and registered handler in generated stubs (`*_pb2_grpc.py`) because registered method flow is not implemented in aysncio yet, since the stub is used for both sync and asyncio, we can't remove generic method flow until we fully implement the registered method flow in asyncio.
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Closes#36371
PiperOrigin-RevId: 629842051
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Closes#36448
PiperOrigin-RevId: 629113962
### NOTE
* We shouldn't merge this PR until GCP cloud functions drops support for Python 3.7 ([Currently scheduled for GCF](https://cloud.google.com/functions/docs/runtime-support#python))
As part of supporting Python 3.12, we're now officially drop support for Python 3.7.
This PR:
* Changed supported Python version from 3.7 to 3.8 in README.
* Replaced distribution test image from `debian:buster` to `debian:bullseye` since the default Python version in buster is 3.7.
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Closes#34450
PiperOrigin-RevId: 622899511
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Discuss thread about this change: [link](https://mail.google.com/mail/u/0/#sent/QgrcJHsBpWNGRlrMktwbppGGfFTVCFLcQgL?compose=new)
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Closes#35849
PiperOrigin-RevId: 607144827
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Closes#35792
PiperOrigin-RevId: 604695910
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Closes#35292
PiperOrigin-RevId: 595188404
### Changes in this PR
* Refactor and remove some Core/C++ dependencies to simplify Python Observability package build process.
* Refactored code to read config at Python layer.
* Enable observability build from source.
* Add observability to run_test.
* Currently it's only enabled in Linux.
* Add error handler in run_test loaders.
* Current framework will always visit modules in test directory then decide which tests to skip.
* Since we're not building Observability for MacOS and Windows this step will fail with error `No module named 'grpc_observability'`.
* After the change we'll just skip those modules.
* We still have `_sanity_test` to make sure all tests are loaded correctly for each platform.
* Remov OC dependency as we're migrating to OTel.
* Also removed trace from testing.
* Note that trace propagation function was also removed because of this.
### Testing
* Passed existing tests.
* Tested locally, able to build observability from source using `GRPC_PYTHON_BUILD_WITH_CYTHON=1 pip install .`.
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Closes#34207
PiperOrigin-RevId: 590258014
Xuan Wang