logging middleware: log trace_id only if sampled (#529)

* logging middleware: log `trace_id` only if sampled

* if not sampled, log it as `trace_id_unsampled`

* update changelog

CHANGELOG.md

@@ -211,6 +211,7 @@
 * [ENHANCEMENT] Expose `InstancesWithTokensCount` and `InstancesWithTokensInZoneCount` in `ring.ReadRing` interface. #516
 * [ENHANCEMENT] Middleware: determine route name in a single place, and add `middleware.ExtractRouteName()` method to allow consuming applications to retrieve the route name. #527
 * [ENHANCEMENT] SpanProfiler: do less work on unsampled traces. #528
+* [ENHANCEMENT] Log Middleware: if the trace is not sampled, log its ID as `trace_id_unsampled` instead of `trace_id`. #529
 * [BUGFIX] spanlogger: Support multiple tenant IDs. #59
 * [BUGFIX] Memberlist: fixed corrupted packets when sending compound messages with more than 255 messages or messages bigger than 64KB. #85
 * [BUGFIX] Ring: `ring_member_ownership_percent` and `ring_tokens_owned` metrics are not updated on scale down. #109

middleware/logging.go

@@ -56,9 +56,11 @@ func NewLogMiddleware(log log.Logger, logRequestHeaders bool, logRequestAtInfoLe
 // logWithRequest information from the request and context as fields.
 func (l Log) logWithRequest(r *http.Request) log.Logger {
 	localLog := l.Log
-	traceID, ok := tracing.ExtractTraceID(r.Context())
+	traceID, ok := tracing.ExtractSampledTraceID(r.Context())
 	if ok {
 		localLog = log.With(localLog, "trace_id", traceID)
+	} else if traceID != "" {
+		localLog = log.With(localLog, "trace_id_unsampled", traceID)
 	}
 
 	if l.SourceIPs != nil {