Update module prometheus/common to v0.21.0

This MR contains the following updates:

Package	Type	Update	Change
github.com/prometheus/common	require	minor	v0.20.0 -> v0.21.0

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Release Notes

prometheus/common

v0.21.0

Compare Source

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vendor/modules.txt

@@ -88,7 +88,7 @@ github.com/prometheus/client_golang/prometheus/internal
 github.com/prometheus/client_golang/prometheus/promhttp
 # github.com/prometheus/client_model v0.2.0
 github.com/prometheus/client_model/go
-# github.com/prometheus/common v0.20.0
+# github.com/prometheus/common v0.21.0
 ## explicit
 github.com/prometheus/common/expfmt
 github.com/prometheus/common/internal/bitbucket.org/ww/goautoneg