Inject active span trace_id into log records

[prim-8]

Summary

• Adds a logging.Filter that pulls the active span's trace_id (from either ddtrace or OTel, whichever provider is active) and stamps it on every LogRecord.
• Updates the format string to surface it as trace_id=<32-hex>.
• Defaults to empty when no tracing is active or no span is current, so the format never KeyErrors.

This enables logs-to-traces correlation in Grafana Explore (and any other UI that runs a regex over the line) for milter log lines — previously the click-through never matched.

Test plan

• Standalone smoke test: filter populates trace_id=<32-hex> inside an active OTel span, clears it outside, no KeyError on weird messages, no KeyError when no provider is active.
• Build the milter image and run a fake mail through it, verify trace_id= field in stdout.
• After merge + bump in the consumer repo, verify Grafana Explore's logs→traces derived-field click-through resolves to a real Tempo trace.

[greptile-apps]

Greptile Summary

Adds a _TraceIdFilter that injects the active span's trace_id into every LogRecord and updates the log format string to surface it, enabling logs-to-traces correlation in Grafana Explore.

• _TraceIdFilter.filter() always sets record.trace_id = \"\" first, so the %(trace_id)s format token never raises KeyError when no span is active or no tracing provider is configured.
• The filter is attached at the handler level (rather than the logger level) so it fires for records from third-party libraries as well as application loggers.
• Imports of ddtrace/opentelemetry are deferred inside the filter method; Python's import cache makes repeated calls cheap.

Confidence Score: 5/5

Safe to merge — the change is additive and isolated to log formatting; the filter always sets a default so the format string is safe even with no active tracing provider.

The logic is straightforward: the filter guards every exit path with a default empty string, the handler-level attachment means all loggers are covered, and Python's module cache keeps the per-record import lookups cheap. Both the ddtrace and OTel branches are handled correctly, including the 0-valued INVALID_TRACE_ID case.

No files require special attention.

Important Files Changed

Filename	Overview
milter/primitivemail_milter.py	Adds _TraceIdFilter logging.Filter that injects the active span's trace_id into every log record for both ddtrace and OTel providers; updates the log format string and attaches the filter to all configured handlers.

_{Reviews (2): Last reviewed commit: "Drop dead-code None guard on get_current..." | Re-trigger Greptile}