otel: add metrics endpoint and scrape target (#886)

Author: burmudar

otel-collector/config.yaml

@@ -20,6 +20,9 @@ extensions:
     endpoint: "localhost:55679"
 
 service:
+  telemetry:
+    metrics:
+      address: ":8888"
   pipelines:
     traces:
       exporters:

prometheus/prometheus_targets.yml

@@ -96,3 +96,8 @@
     job: node-exporter
   targets:
     - node-exporter:9100
+- labels:
+    nodename: "sourcegraph-docker-compose-host"
+    job: otel-collector
+  targets:
+    - otel-collector:8888