Update module github.com/prometheus/client_golang to v1.20.4

Author: renovate[bot]

go.mod

@@ -6,7 +6,7 @@ require (
 	github.com/docker/docker v20.10.3+incompatible // indirect
 	github.com/docker/libnetwork v0.8.0-dev.2.0.20210125164233-b3507428be5b
 	github.com/ishidawataru/sctp v0.0.0-20210204083202-2bdd86bb9a4d // indirect
-	github.com/prometheus/client_golang v1.12.2
+	github.com/prometheus/client_golang v1.20.4
 	github.com/sirupsen/logrus v1.8.0 // indirect
 	github.com/vishvananda/netlink v1.1.0 // indirect
 	github.com/vishvananda/netns v0.0.0-20210104183010-2eb08e3e575f // indirect