* Latency in 50 percentile:

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Verified that when following User Guide, we're able to collect metrics
and export to GCP.
Images are available at:
*
`us-docker.pkg.dev/grpc-testing/examples/csm-o11y-example-python-client:latest`
*
`us-docker.pkg.dev/grpc-testing/examples/csm-o11y-example-python-server:latest`
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Closes#36448
Address comments from design review meeting, mainly:
* Use `OpenTelemetryPlugin` as public API.
* Use keyword args to build plugin.
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Closes#36094
Allow start observability globally with a new API `start_open_telemetry_observability`.
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Closes#35932
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Closes#35792
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