Merge branch 'renovate/github.com-prometheus-client_golang-1.x' into 'master'

Update module prometheus/client_golang to v1.9.0

See merge request !6

go.mod

@@ -7,7 +7,7 @@ require (
 	github.com/jhillyerd/enmime v0.8.3
 	github.com/modern-go/concurrent v0.0.0-20180306012644-bacd9c7ef1dd // indirect
 	github.com/oschwald/geoip2-golang v1.4.0
-	github.com/prometheus/client_golang v1.8.0
+	github.com/prometheus/client_golang v1.9.0
 	golang.org/x/sync 09787c993a3a
 	gopkg.in/check.v1 v1.0.0-20190902080502-41f04d3bba15 // indirect
 )