Code Review

This pull request introduces tracing for AFE latency by adding an afe_latency attribute to spans. To support this, the logic for processing server-timing headers in HeaderInterceptor has been significantly refactored. The new structure is much cleaner, separating metric recording by system (OpenCensus, Built-in Metrics, Span Attributes), which improves readability and maintainability. The use of Float for latency values also gracefully handles cases where headers are missing. The corresponding test changes are appropriate. Overall, this is a great improvement.