Stamp trace_id and span_id on every log line inside a span

A processor in the shared chain reads the active span context and adds
its IDs as lowercase hex, so both structlog and stdlib records carry
them. The lowercase `trace_id` name is what the provisioned Loki derived
field's case-sensitive regex matches; outside a span nothing is added.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

common/logging_config.py

@@ -2,6 +2,7 @@ import logging
 import sys
 
 import structlog
+from opentelemetry import trace
 
 # Loggers that install their own handlers (uvicorn's dictConfig runs before the
 # app is imported). Their handlers are stripped so records propagate to the
@@ -19,6 +20,22 @@ def _add_service(service_name: str):
     return processor
 
 
+def _add_trace_context(logger, method_name, event_dict):
+    """Stamp the active span's IDs on the record, as lowercase hex.
+
+    The name `trace_id` is load-bearing: Grafana's Loki derived field matches
+    the case-sensitive regex trace[_-]?id["=: ]+([a-fA-F0-9]+), so OTel's own
+    `otelTraceID` would never become a link. Outside a span (startup, the
+    consumers' idle loop) the context is invalid and nothing is added, rather
+    than an all-zeros ID that points at no trace.
+    """
+    ctx = trace.get_current_span().get_span_context()
+    if ctx.is_valid:
+        event_dict["trace_id"] = trace.format_trace_id(ctx.trace_id)
+        event_dict["span_id"] = trace.format_span_id(ctx.span_id)
+    return event_dict
+
+
 def configure_logging(service_name: str) -> structlog.stdlib.BoundLogger:
     """One JSON object per line on stdout, for structlog and stdlib loggers alike.
 
@@ -32,6 +49,7 @@ def configure_logging(service_name: str) -> structlog.stdlib.BoundLogger:
         structlog.stdlib.add_logger_name,
         structlog.processors.TimeStamper(fmt="iso", utc=True),
         _add_service(service_name),
+        _add_trace_context,
     ]
 
     structlog.configure(