Update module prometheus/client_golang to v1.9.0

This MR contains the following updates:

Package	Type	Update	Change
github.com/prometheus/client_golang	require	minor	v1.8.0 -> v1.9.0

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Release Notes

prometheus/client_golang

v1.9.0

Compare Source

• [FEATURE] NewPidFileFn helper to create process collectors for processes whose PID is read from a file. #​804
• [BUGFIX] promhttp: Prevent endless loop in InstrumentHandler... middlewares with invalid metric or label names. #​823

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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
 )