Register quota metrics in GCP proxy

When a quota request is rejected, increment the metric counter by one.

Also makes both frontend and backend metrics singleton because all the fields they have a static.

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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=185146804

java/google/registry/proxy/metric/BackendMetrics.java

@@ -26,8 +26,10 @@ import google.registry.util.NonFinalForTesting;
 import io.netty.handler.codec.http.FullHttpRequest;
 import io.netty.handler.codec.http.FullHttpResponse;
 import javax.inject.Inject;
+import javax.inject.Singleton;
 
 /** Backend metrics instrumentation. */
+@Singleton
 public class BackendMetrics {
 
   // Maximum request size is defined in the config file, this is not realistic and we'd be out of