feat(gax): request-level custom HTTP/gRPC headers

[xlai20]

Overview

Adds support for request-level custom HTTP/gRPC headers via RequestOptionsBuilder::with_custom_header, allowing developers to inject tracking or tracing headers (e.g., x-client-tracking-id, X-B3-TraceId) on individual API calls without signature changes.

DD: go/rust-sdk-custom-header
Part of #5997

Key Changes

• google-cloud-gax:
  • Added default method with_custom_header<K, V>(mut self, name: K, value: V) -> Self to RequestOptionsBuilder.
• google-cloud-gax-internal:
  • Updated ReqwestClient::request() to extract custom headers and enforce a "System-Wins" precedence policy.
• Tests:
  • Added unit tests for RequestOptionsBuilder::with_custom_header.
  • Added HTTP transport wire emission and precedence tests in http_custom_header.rs (via echo_server).
  • Integration tests will be added later in google-cloud-storage upon version bump in next PR.

[gemini-code-assist]

Code Review

This pull request introduces support for injecting custom HTTP headers into requests via RequestOptionsBuilder::with_custom_header, along with comprehensive integration and unit tests. The review feedback highlights two key issues: first, the current logic for merging system headers silently discards multi-valued headers due to how HeaderMap iteration works; second, swallowing conversion errors when parsing custom header names and values violates the "Demand Explosive Correctness" principle, and should instead fail loudly.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 96.24%. Comparing base (35b9590) to head (7a54bb5).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #6260      +/-   ##
==========================================
- Coverage   96.24%   96.24%   -0.01%     
==========================================
  Files         280      280              
  Lines       72146    72195      +49     
==========================================
+ Hits        69440    69485      +45     
- Misses       2706     2710       +4     

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