Merge pull request #181 from nginxinc/dependabot/go_modules/github.com/prometheus/common-0.21.0

Bump github.com/prometheus/common from 0.20.0 to 0.21.0

Author: ciarams87

go.mod

@@ -6,5 +6,5 @@ require (
 	github.com/golangci/golangci-lint v1.39.0
 	github.com/nginxinc/nginx-plus-go-client v0.8.0
 	github.com/prometheus/client_golang v1.10.0
-	github.com/prometheus/common v0.20.0
+	github.com/prometheus/common v0.21.0
 )