Prometheus, which runs as part of Linkerd's control plane, retains data for approximately 6 hours. If further data retention is required, the metrics can be exported to a data store such as the Elasticsearch, Logstash, and Kibana (ELK) stack or to a dedicated Prometheus instance. 

The Prometheus federation API or ServiceMonitors can be used to copy data using the /federate path from Linkerd Prometheus to the dedicated Prometheus store. 

Alternatively, you can call the federation API directly, and then it can be ingested by a Kafka stream, which can then dump it to an ELK stack, like so:

 $ curl -G 
 --data-urlencode 'match[]={job="linkerd-proxy"}' 
 --data-urlencode 'match[]={job="linkerd-controller"}' 
 http://linkerd-prometheus.linkerd.svc.cluster.local:9090/federate

From the preceding curl command, you can use an external name through Ingress to route traffic to the linkerd-prometheus microservice if the /federate path needs to be scraped external to the cluster. 

There can be multiple Linkerd proxies running within a data plane and a control plane. You can scrape metrics directly from a Linkerd proxy like so:

$ kubectl -n linkerd port-forward 
  (kubectl -n linkerd get pods 
 -l linkerd.io/control-plane-ns=linkerd 
 -o jsonpath='{.items[0].metadata.name}') 
 4191:4191

From your local machine, after you run the preceding kubectl command, you can run the following curl command and access the Prometheus federated metrics datastore:

$ curl localhost:4191/metrics

Next, let's dive into the Linkerd sidecar proxy and how to enable the debugging flag. This will provide detailed views into the microservice it's attached to.