metropolis: implement Metrics Service

This is the first pass at a Metrics Service. It currently consists of an
HTTP reverse proxy which authenticates incoming connections using the
Cluster CA and certificates, and passes these connections over to a
locally running node_exporter.

In the future more exporters will be added, and we will likely also run
our own exporter for Metropolis-specific metrics.

Change-Id: Ibab52aa303965dd7d975f5035f411d1c56ad73e6
Reviewed-on: https://review.monogon.dev/c/monogon/+/1816
Tested-by: Jenkins CI
Reviewed-by: Leopold Schabel <leo@monogon.tech>
diff --git a/metropolis/node/core/roleserve/roleserve.go b/metropolis/node/core/roleserve/roleserve.go
index 68b9f59..f0fa273 100644
--- a/metropolis/node/core/roleserve/roleserve.go
+++ b/metropolis/node/core/roleserve/roleserve.go
@@ -93,6 +93,7 @@
 	nodeMgmt     *workerNodeMgmt
 	clusternet   *workerClusternet
 	hostsfile    *workerHostsfile
+	metrics      *workerMetrics
 }
 
 // New creates a Role Server services from a Config.
@@ -163,6 +164,10 @@
 		clusterDirectorySaved: &s.clusterDirectorySaved,
 	}
 
+	s.metrics = &workerMetrics{
+		curatorConnection: &s.CuratorConnection,
+	}
+
 	return s
 }
 
@@ -232,6 +237,7 @@
 	supervisor.Run(ctx, "nodemgmt", s.nodeMgmt.run)
 	supervisor.Run(ctx, "clusternet", s.clusternet.run)
 	supervisor.Run(ctx, "hostsfile", s.hostsfile.run)
+	supervisor.Run(ctx, "metrics", s.metrics.run)
 	supervisor.Signal(ctx, supervisor.SignalHealthy)
 
 	<-ctx.Done()