feat(storage): Client Setup & REST feature tracking

[v-pratap]

No description provided.

[gemini-code-assist]

Code Review

This pull request introduces a FeatureTracker mechanism to track and report client-side optimizations and configuration choices in the Google Cloud Storage C++ client via a custom x-goog-storage-cpp-features header. The feedback highlights two key improvements: first, optimizing EncodeFeatureTrackerBitmask by replacing std::vectorstd::uint8_t with std::string to leverage Small String Optimization (SSO) and avoid heap allocations on the hot path; second, correcting a test assertion in stub_test.cc where ElementsAre is incorrectly used on a string value, which would cause a compilation or matching failure.

[gemini-code-assist]

Using std::vector<std::uint8_t> to accumulate the bytes for the bitmask causes a heap allocation on every call to EncodeFeatureTrackerBitmask (which is invoked on every REST request via AddHeaders).

Since the maximum size of the bitmask is 4 bytes, we can use std::string instead. Due to Small String Optimization (SSO), std::string will avoid heap allocation entirely, improving performance on the hot path.

  std::string bytes;
  for (int i = 0; i < 4; ++i) {
    auto b = static_cast<char>((mask >> (24 - i * 8)) & 0xFF);
    if (!bytes.empty() || b != 0) {
      bytes.push_back(b);
    }
  }
  return Base64Encode(bytes);

[gemini-code-assist]

Since RestRequest::headers() returns a collection of std::pair<std::string, std::string>, the value of the pair is a std::string, not a container. Using ElementsAre here is incorrect and will cause a compilation or matching failure. It should match the string directly.

              Contains(Pair(kFeatureTrackerHeaderName,
                            tracker->HeaderValue())))