Add GCP OpenTelemetry Cloud Run worker sample

[eamsden]

What changed

• Add a Java sample that runs a continuously polling Temporal worker in a Cloud Run worker pool.
• Configure GcpOpenTelemetryPlugin to export standard OTLP/gRPC to a Google-Built OpenTelemetry Collector sidecar on localhost:4317.
• Export cumulative metrics directly to Google Managed Service for Prometheus without collector batching, while retaining a dedicated five-second trace batch and the supported Telemetry API path.
• Add a worker-pool manifest with collector startup dependency and health probes, Secret Manager-backed collector configuration, bounded worker shutdown, and post-termination telemetry flushing.
• Document required Google Cloud APIs, least-privilege runtime IAM, deployment, signal verification, the coordinated 60-second SDK default, custom interval constraints, and shutdown verification.

Validation

• Clean Gradle build and installDist passed against immutable sdk-java commit 4bc57576, including the workflow test and a regression assertion for the 60-second plugin default.
• otelcol-google:0.156.0 validate accepted collector-config.yaml.
• Both YAML files parse successfully and git diff --check passes.
• Docker BuildKit's static Dockerfile check completed with no warnings.
• The related SDK Cloud Run worker-pool E2E verified the same 60-second cadence, unbatched cumulative-metrics pipeline, dedicated trace batch, workflow execution, telemetry export, and clean shutdown without duplicate-series or exporter errors. This sample was not separately deployed because the SDK artifact is not yet published.

Expected blocker

io.temporal:temporal-gcp is unreleased. The sample intentionally uses 1.37.0-SNAPSHOT as a placeholder and supports Gradle composite substitution for local verification. A normal Maven Central build and the production Docker build remain blocked until the first release containing the module; this PR should remain draft until that dependency can be replaced with the released version.

The draft PR's unit checks confirm this expected state: they reach :gcp-opentelemetry:compileJava and fail only because Maven Central does not contain the temporal-sdk, temporal-envconfig, or temporal-gcp 1.37.0-SNAPSHOT artifacts. Formatting, wrapper validation, dependency submission, SAST, and CLA checks pass.