[CPLAT-8549] Improve access log : record trace sampled

[ku524]

Summary

Add a new access log formatter that extracts the sampled flag from the traceparent header. Use %TRACEPARENT_SAMPLED% command in access log format.

Changes

• New Formatter: TraceparentSampledFormatter class
  • Parses W3C Trace Context traceparent header
  • Extracts LSB (sampled bit) from trace-flags and returns "true" or "false"
  • Supports both Request and Response headers

Technical Details

• traceparent header format: {version}-{trace-id}-{parent-id}-{trace-flags}
• Validates minimum 55 characters (version(2) + trace_id(32) + span_id(16) + flags(2) +
  delimiters)
• Parses last 2 hex characters; returns sampled if LSB is 1

Test Plan

• sampled=01 → returns "true"
• sampled=00 → returns "false"
• Missing header → returns nullopt
• Invalid format (insufficient length) → returns nullopt
• Invalid hex value → returns nullopt
• Response header support
• formatValueWithContext tests

Usage Example

  access_log:
    - name: envoy.access_loggers.file
      typed_config:
        "@type": type.googleapis.com/envoy.extensions.access_loggers.file.v3.FileAccessLog
        path: /dev/stdout
        log_format:
          text_format: "[%START_TIME%] %TRACEPARENT_SAMPLED%\n"