Write usage logs when diverting to spanner

[n-h-diaz]

Summary

Enables consistent usage logging (WriteUsageLog), x-response-id header generation, and request metadata propagation (X-Surface HTTP header) across both default and dispatcher execution paths for V2 Observation queries.

Key Changes

• Unified Usage Logging & Header Generation (handler_v2.go):

  • Refactored V2Observation so that the diversion path assigns v2Resp and queryType rather than returning early.
  • Both default and diverted paths now share the post-execution block that generates responseId, sets x-response-id in the gRPC response headers, extracts surface and toRemote metadata from ctx and calls log.WriteUsageLog.

• Standalone GetQueryType Helper (v2/observation/observation.go):

  • Extracted query-type classification logic into an exported helper function GetQueryType.
  • ObservationCore now computes queryType := GetQueryType(in) upfront and returns it across all branches.
  • In V2Observation, v2observation.GetQueryType(in) is called when requests are diverted.

• Context Propagation & X-Surface Forwarding (remote/client.go & remote/datasource.go):

  • Updated all 9 RPC methods on RemoteClient to accept ctx context.Context.
  • Added a getSurface(ctx) helper to extract x-surface from the request context and forward it to util.FetchRemote(...).
  • Updated RemoteDataSource to pass ctx to all RemoteClient method invocations.

• Test Coverage:

  • Added new unit tests in remote/client_test.go using an httptest.Server to verify X-Surface and X-Remote HTTP header forwarding through RemoteDataSource and RemoteClient.
  • Updated existing tests in handler_v2_test.go and observation_test.go to validate usage logging and GetQueryType classification.

[gemini-code-assist]

Code Review

This pull request refactors the V2 observation handler to support query diversion to a dispatcher, propagates context through the remote client to forward surface metadata headers, and extracts query type determination into a reusable helper function with accompanying tests. The review feedback suggests simplifying the V2 observation handler by consolidating redundant calls to util.GetMetadata(ctx) at the beginning of the function.

[gmechali]

Thanks for fixing this!

Curious, were you abnle to run this local, or deploy this image, and confirm the logs all showed up as expevted?

[n-h-diaz]

Thanks for fixing this!

Curious, were you abnle to run this local, or deploy this image, and confirm the logs all showed up as expevted?

Thanks for the review! Tested locally that responses are getting logged - there's one diff where we aren't returning place types for containedInPlace requests in the spanner response, so it's not showing up in the log entry, but this is unrelated to the logger itself (is a diff in the response), so will address as a followup