Update opencensus-go-exporter-stackdriver to v0.13.4. (#252)

Author: qingling128

go.mod

@@ -3,7 +3,7 @@ module github.com/Stackdriver/stackdriver-prometheus-sidecar
 require (
 	cloud.google.com/go v0.49.0
 	contrib.go.opencensus.io/exporter/prometheus v0.1.0
-	contrib.go.opencensus.io/exporter/stackdriver v0.12.8
+	contrib.go.opencensus.io/exporter/stackdriver v0.13.4
 	github.com/Azure/azure-sdk-for-go v36.2.0+incompatible // indirect
         github.com/Azure/go-autorest v13.3.0+incompatible // indirect
 	github.com/Azure/go-autorest/autorest/adal v0.8.0 // indirect
@@ -48,7 +48,7 @@ require (
 	github.com/samuel/go-zookeeper v0.0.0-20190801204459-3c104360edc8 // indirect
 	github.com/spaolacci/murmur3 v1.1.0 // indirect
 	github.com/spf13/pflag v1.0.5 // indirect
-	go.opencensus.io v0.22.2
+	go.opencensus.io v0.22.4
 	golang.org/x/crypto v0.0.0-20191128160524-b544559bb6d1 // indirect
 	golang.org/x/net v0.0.0-20191126235420-ef20fe5d7933 // indirect
 	golang.org/x/oauth2 v0.0.0-20191122200657-5d9234df094c // indirect