metropolis/node/core/metrics: implement http_sd discovery endpoint

We provide prometheus metrics but dont have a way to discover all nodes,
this change implements a new http endpoint: /discovery. It implements the
http_sd api and returns all current cluster nodes including their roles as
label.

Change-Id: I931a88e2afb285482d122dd059c96f9ebfab4052
Reviewed-on: https://review.monogon.dev/c/monogon/+/1934
Tested-by: Jenkins CI
Reviewed-by: Serge Bazanski <serge@monogon.tech>
diff --git a/metropolis/node/core/roleserve/roleserve.go b/metropolis/node/core/roleserve/roleserve.go
index ddc5811..02bc510 100644
--- a/metropolis/node/core/roleserve/roleserve.go
+++ b/metropolis/node/core/roleserve/roleserve.go
@@ -167,6 +167,7 @@
 
 	s.metrics = &workerMetrics{
 		curatorConnection: &s.CuratorConnection,
+		localRoles:        &s.localRoles,
 	}
 
 	return s