Summary

This change adds substitution formatter support to
equest_headers_to_add for HTTP service-based integrations that previously treated header values as static strings.

It introduces a shared helper, HttpServiceHeadersApplicator, and wires it into:

• OpenTelemetry HTTP metrics exporter
• Zipkin tracer HTTP collector path

As a result, configured headers can use formatter expressions and are evaluated at apply-time rather than only at construction-time.

Motivation

Some HTTP service configurations need dynamic header values. Existing code paths in these integrations only copied raw string values and did not support formatter evaluation.

What changed

• Added HttpServiceHeadersApplicator:
  • source/common/http/http_service_headers.h
  • source/common/http/http_service_headers.cc
  • http_service_headers_lib in source/common/http/BUILD
• Updated OpenTelemetry stat sink HTTP exporter to use the shared helper.
• Updated Zipkin tracer HTTP collector path to use the shared helper.
• Added required build deps in OpenTelemetry and Zipkin BUILD files.

Behavior

• Static headers continue to work as before.
• HttpService header values can now use substitution formatter expressions.
• Formatter parse/setup errors fail initialization with a clear exception.

Testing

• CI/build validation in PR.
• Manual verification by configuring formatter expressions in
  equest_headers_to_add and checking resolved outbound headers.