Add example of how to scrape prometheus from the agent. (#75)

Co-authored-by: Arne Mejlholm <arne.mejlholm@subaio.com>

Author: mejlholm

README.md

@@ -306,6 +306,14 @@ docker run --rm -p 8083:8083 -p 8093:8093 \
 **Note:** The `WORKDIR` of the proxy and agent images is `/app`, so make sure to use `/app` as the base directory in the
 target for `--mount` options.
 
+## Scraping a prometheus instance
+It's possible to scrape an existing prometheus server using the /federate endpoint. 
+This enables using the existing service discovery features already built into prometheus. 
+
+An example config can be found in
+[federate.conf](https://raw.githubusercontent.com/pambrose/prometheus-proxy/master/examples/federate.conf)
+
+
 ## Grafana
 
 [Grafana](https://grafana.com) dashboards for the proxy and agent

examples/federate.conf

@@ -0,0 +1,14 @@
+agent {
+  pathConfigs: [
+    {
+      name: "Federate metrics"
+      path: federate_metrics
+      url: "http://prometheus:9090/federate?match[]={job=~'.*'}"
+    },
+    {
+      name: "Agent metrics"
+      path: agent_metrics
+      url: "http://localhost:8083/metrics"
+    }
+  ]
+}