chore: add missing Prometheus service to grafana-tempo monitoring exa…

…mple.

  Prometheus is needed to collect metrics and serve them to grafana.
  While we don't have many metrics yet emitted directly by Amaru, we are
  able to automatically derive some from spans using the OpenTelemetry
  collector with the 'spanmetrics' collector/plugin.

monitoring/grafana-tempo/docker-compose.yml

@@ -10,6 +10,17 @@ services:
       - "55679:55679"
       - "13133:13133"
 
+  prometheus:
+    image: prom/prometheus:v3.0.1
+    command:
+      [ "--web.enable-otlp-receiver"
+      , "--config.file=/etc/prometheus/prometheus.yml"
+      ]
+    volumes:
+      - "./prometheus.yml:/etc/prometheus/prometheus.yml"
+    ports:
+      - "9090:9090"
+
   tempo:
     image: grafana/tempo:latest
     ports: