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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