Initial commit of the OpenTelemetry trace exposition

[rakyll]

Refactor telemetry configuration and resolve deprecated GCP metadata calls:

• Replace deprecated metadata.ProjectID() and metadata.OnGCE() with context-aware versions.
• Update telemetry.SetTraceProvider to accept variadic otlptracegrpc.Options and remove the hardcoded endpoint parameter.
• Add telemetry.WithGCPCredentials() helper to encapsulate GCP-specific credentials.
• Isolate GCP-specific server telemetry configuration into a new serve_gcp.go file.

Fix premature gRPC stream cancellation in the harness execution clients:

• Update the client receive loop to drain the stream until io.EOF after receiving the HarnessEnd frame, preventing CANCELLED errors in telemetry.
• Encapsulate the stream draining and message dispatching logic into a shared drainStream helper in a new stream.go file.
• Clean up unused io imports in antigravity.go and substrate.go.

Follow-up:

• End-to-end trace context propagation to HarnessService deployments
• End-to-end trace context propagation to Substrate calls