feat(storage): add feature tracker for multistream feature

[v-pratap]

No description provided.

[gemini-code-assist]

Code Review

This pull request introduces a client-side feature tracking mechanism (FeatureTracker) to report adopted optimizations and configuration choices via the x-goog-storage-cpp-features header in both gRPC and REST paths. While the overall implementation is solid, there are critical issues with how the header is attached. Specifically, adding the tracker to fixed_metadata in CreateDecoratedStubs makes the header static, which violates per-request options and fails to capture dynamic features. Additionally, the asynchronous streaming path in OpenObject needs to respect the EnableFeatureReportsOption rather than adding the header unconditionally. Addressing these issues will also allow the removal of the now-redundant FeatureTrackerInStorageMetadata test.

[gemini-code-assist]

Adding the feature tracker header to fixed_metadata in CreateDecoratedStubs makes it a static header for the lifetime of the client. This has two major drawbacks:

1. It violates the request-specific EnableFeatureReportsOption(false) because StorageMetadata's fixed_metadata is appended unconditionally to every request, bypassing any per-request options.
2. It cannot support any dynamic or operation-level features that might be registered on the tracker later, as the header value is copied once during stub creation.

Since ApplyQueryParameters (for gRPC) and AddHeaders (for REST) already dynamically and correctly append this header while respecting request-specific options, we should remove the feature tracker from fixed_metadata in CreateDecoratedStubs.

  std::multimap<std::string, std::string> fixed_metadata;

[gemini-code-assist]

The asynchronous streaming RPC path currently adds the feature tracker header unconditionally if a tracker is present. It should respect the EnableFeatureReportsOption from the request-specific options to allow users to disable feature reporting for specific operations.

  bool const enable_reports =
      !options.has<storage::EnableFeatureReportsOption>() ||
      options.get<storage::EnableFeatureReportsOption>();
  if (enable_reports && tracker) {
    auto v = tracker->HeaderValue();
    if (!v.empty()) {
      context->AddMetadata(storage::internal::kFeatureTrackerHeaderName,
                           std::move(v));
    }
  }

[gemini-code-assist]

This test is no longer needed because the feature tracker header is no longer added to StorageMetadata's fixed_metadata. The dynamic metadata path is already covered by ApplyQueryParametersWithFeatureTracker in configure_client_context_test.cc.