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We already have the option to use Xds credentials based on `secure_mode`
flag, this PR add the ability to create Xds server based on the same
flag.
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* 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
PiperOrigin-RevId: 629113962
Address comments from design review meeting, mainly:
* Use `OpenTelemetryPlugin` as public API.
* Use keyword args to build plugin.
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Closes#36094
PiperOrigin-RevId: 615807264
Allow start observability globally with a new API `start_open_telemetry_observability`.
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Closes#35932
PiperOrigin-RevId: 612639020
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Closes#35792
PiperOrigin-RevId: 604695910
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