feat(opentelemetry sink): add gRPC transport support

[thomasqueirozb]

Summary

Adds gRPC transport to the OpenTelemetry sink, complementing the existing HTTP transport. The sink now supports protocol.type = "grpc" using tonic over HTTP/2, with support for TLS, compression, batching, and retry logic matching the source's gRPC implementation.

Vector configuration

sinks:
  otel_grpc:
    type: opentelemetry
    inputs: [...]
    protocol:
      type: grpc
      endpoint: http://otel-collector:4317

How did you test this PR?

• Unit tests in src/sinks/opentelemetry/grpc.rs covering config parsing and URI scheme handling
• E2e test matrix extended with vector_grpc.yaml config (telemetrygen → OTel collector → Vector gRPC sink → OTel collector sink → file)
• Integration test in src/sinks/opentelemetry/integration_tests.rs (gated on opentelemetry-integration-tests feature) that sends OTLP log events to a live OTel collector via gRPC

Change Type

• Bug fix
• New feature
• Dependencies
• Non-functional (chore, refactoring, docs)
• Performance

Is this a breaking change?

• Yes
• No

Does this PR include user facing changes?

• Yes. Please add a changelog fragment based on our guidelines.
• No. A maintainer will apply the no-changelog label to this PR.

References

• Closes: Support gRPC in the OTEL sink #23062

[OliviaShoup]

thanks for the PR! i've created an editorial review card, and someone on the docs team will take a look soon: https://datadoghq.atlassian.net/browse/DOCS-13836

[sakateka]

Hi!

Thank you for the great work on supporting the gRPC transport for the OpenTelemetry sink.

I decided to try using the gRPC sink in my setup. After making a few adjustments, everything worked out — now I’m using this new transport to send logs in my env. That’s why I want to help with the current PR and am attaching my modifications as a PR to this branch.

#25757

[pront]

Hi!

Thank you for the great work on supporting the gRPC transport for the OpenTelemetry sink.

I decided to try using the gRPC sink in my setup. After making a few adjustments, everything worked out — now I’m using this new transport to send logs in my env. That’s why I want to help with the current PR and am attaching my modifications as a PR to this branch.

#25757

@sakateka thanks! @thomasqueirozb can you review and pull fixes into this PR? Let's drive this to completion.