cloud: enable prometheus server

This makes cloud components start a Prometheus server on a separate port
(:4243 by default) using either gRPC TLS credentials or plain HTTP.

The Prometheus server currently only export the standard Go/Process
metrics that the default Prometheus library exports.

Change-Id: I9f2ae20c34446c0e10a946d4a93251764f5d2fce
Reviewed-on: https://review.monogon.dev/c/monogon/+/1599
Reviewed-by: Leopold Schabel <leo@monogon.tech>
Tested-by: Jenkins CI
diff --git a/go.mod b/go.mod
index 9717f10..a9ab662 100644
--- a/go.mod
+++ b/go.mod
@@ -335,7 +335,7 @@
 	github.com/pkg/xattr v0.4.1 // indirect
 	github.com/pmezard/go-difflib v1.0.0 // indirect
 	github.com/pquerna/cachecontrol v0.0.0-20171018203845-0dec1b30a021 // indirect
-	github.com/prometheus/client_golang v1.12.1 // indirect
+	github.com/prometheus/client_golang v1.12.1
 	github.com/prometheus/client_model v0.2.0 // indirect
 	github.com/prometheus/common v0.34.0 // indirect
 	github.com/prometheus/procfs v0.7.3 // indirect