Fix `readme.rst`.
Tested locally by running `python setup.py doc`, no more errors from observability.
Only error is related to grpc_status:
```
WARNING: autodoc: failed to import module 'rpc_status' from module 'grpc_status'; the following exception was raised:
cannot import name 'status_pb2' from 'google.rpc' (unknown location)
```
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Closes#35988
PiperOrigin-RevId: 609820414
Actually build O11y artifacts.
### Testing
* Manually installed the `.whl` and verified it's working locally (For Python 3.8 + Linux).
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Closes#35578
PiperOrigin-RevId: 600566829
Xuan Wang