Add GCP OpenTelemetry plugin

[eamsden]

Summary

• add temporalio.contrib.gcp.OpenTelemetryPlugin, a thin GCP-focused wrapper around the existing OpenTelemetry plugin
• export workflow traces and Core metrics over OTLP/gRPC to a local collector sidecar, with Cloud Run-aware service and revision defaults
• keep tracer-provider and runtime ownership explicit, including flush/shutdown helpers and worker-stop opt-in
• add the gcp-opentelemetry dependency extra, usage documentation, and focused tests

The worker does not use Google Cloud clients, exporters, or resource detection directly. The collector is responsible for authenticating, enriching telemetry with GCP resource attributes, and exporting it to Google Cloud.

Related implementations

• Java: Add Google Cloud OpenTelemetry plugin sdk-java#2955
• Go: Add Google Cloud OpenTelemetry plugin sdk-go#2467

Validation

• 59 focused and regression tests passed
• ruff check --select I .
• ruff format --check .
• pyright
• mypy
• basedpyright
• documentation generation
• uv lock --check
• default plugin construction/shutdown smoke test