Update module dependencies

Author: bvwells

go.mod

@@ -1,9 +1,9 @@
 module contrib.go.opencensus.io/exporter/prometheus
 
 require (
-	github.com/google/go-cmp v0.5.5
-	github.com/prometheus/client_golang v1.9.0
-	github.com/prometheus/statsd_exporter v0.20.0
+	github.com/google/go-cmp v0.5.6
+	github.com/prometheus/client_golang v1.11.0
+	github.com/prometheus/statsd_exporter v0.21.0
 	go.opencensus.io v0.23.0
 )