External Prometheus integration

To export data from Prometheus to another full-fledged metrics backend, you can either pull data directly from Linkerd proxies or federate data to a dedicated Prometheus cluster. You can also use Prometheus APIs to extract data from the local Prometheus store to your dedicated Prometheus data store. Let's get started:

1. For example, call the federation API directly:

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

response_latency_ms_bucket{authority="webapp.linkerd-lab.svc.cluster.local:7000",control_plane_ns="linkerd",deployment="traffic",direction="outbound",dst_control_plane_ns="linkerd",dst_deployment="webapp",dst_namespace="linkerd...