fix: restore OTLP metrics ingestion into Prometheus

Prometheus 3.x replaced the otlp-write-receiver feature flag with
--web.enable-otlp-receiver; with the old flag the collector's metrics
export 404ed and all OTLP metrics were silently dropped (surfaced by the
new demo). Also add a Loki derived field so trace_id arriving as OTLP
structured metadata links to Tempo, not just trace ids in the log body.

Author: simonvanlierde

compose.yml

@@ -62,7 +62,9 @@ services:
       - --storage.tsdb.path=/prometheus
       - --storage.tsdb.retention.time=30d
       - --web.enable-remote-write-receiver
-      - --enable-feature=native-histograms,otlp-write-receiver,exemplar-storage
+      # Prometheus 3.x: OTLP ingestion is its own flag, no longer a feature flag.
+      - --web.enable-otlp-receiver
+      - --enable-feature=native-histograms,exemplar-storage
     logging: *default-logging
 
   grafana:

config/grafana/datasources.yaml

@@ -25,6 +25,12 @@ datasources:
           matcherRegex: "[tT]race[_-]?[iI][dD][\"=:\\s]+([A-Fa-f0-9]+)"
           url: "$${__value.raw}"
           datasourceUid: tempo
+        # Same, for OTLP logs where trace_id is structured metadata, not body text.
+        - name: trace_id (otlp)
+          matcherType: label
+          matcherRegex: trace_id
+          url: "$${__value.raw}"
+          datasourceUid: tempo
 
   - name: Tempo
     type: tempo