fix(spanner): avoid double grpc-gcp wrapping for directpath fallback

[rahul2393]

Fixes DirectPath fallback channel construction so each path gets only one grpc-gcp layer.

Before:

  GcpManagedChannel -> ChannelRef -> inner GcpManagedChannel -> ChannelRef

After:

  GcpFallbackChannel
    -> DirectPath GcpManagedChannel
    -> CloudPath GcpManagedChannel

Impact:

  432 ChannelRef -> 48 ChannelRef
  54 GcpManagedChannel -> 6 GcpManagedChannel

Internal reference: go/grpc-gcp-directpath-fixes

[gemini-code-assist]

Code Review

This pull request refactors GapicSpannerRpc to conditionally apply the gRPC GCP extension and updates FallbackChannelBuilder to use generic ManagedChannelBuilder types. It also adds tests that use reflection to monitor gRPC channel creation. The review feedback highlights a missing method definition for createBaseChannelProviderBuilder and potential inconsistencies in its application. Additionally, a correction was suggested for the test configuration to ensure that GCP fallback is explicitly enabled, preventing tests from being skipped.

[gemini-code-assist]

The method createBaseChannelProviderBuilder is called here, but its definition is not included in the pull request. Additionally, the call site at line 371 (visible in context but not in the diff) still uses createChannelProviderBuilder. If createBaseChannelProviderBuilder is a new method intended to provide a base configuration without the GCP extension, please ensure its definition is included and consider if other call sites should also be updated for consistency.

[rahul2393]

It's there in line 746 createBaseChannelProviderBuilder

[gemini-code-assist]

The SpannerOptions builder in createDirectPathFallbackObjectCountOptions does not explicitly enable GCP fallback. Since isEnableGcpFallback() defaults to false in SpannerOptions, the assumeTrue checks in the tests (e.g., at line 1442 and 1465) will likely cause these tests to be skipped. Explicitly enabling it here ensures the tests actually execute the DirectPath fallback logic.

Suggested change

	.setEnableDirectAccess(true)
	.setHost("http://localhost:1")
	.setEnableDirectAccess(true)
	.setEnableGcpFallback(true)
	.setHost("http://localhost:1")

[rahul2393]

Default is true