Reduce duplicate code in the servlets

Currently, all 4 servlets (backend, frontend, pubapi, tools) have duplicates of
the same exact code.

That's an anti-pattern!

Created a ServletBase they can all extend which has the duplicate code.

As a bonus, the tools servlet now runs the metric reporter, meaning tool
related metrics will now be reported!

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=219792176

java/google/registry/module/pubapi/PubApiComponent.java

@@ -47,12 +47,12 @@ import javax.inject.Singleton;
       CredentialModule.class,
       CustomLogicFactoryModule.class,
       DummyKeyringModule.class,
-      PubApiRequestComponentModule.class,
       Jackson2Module.class,
       KeyModule.class,
       KeyringModule.class,
       KmsModule.class,
       NetHttpTransportModule.class,
+      PubApiRequestComponentModule.class,
       ServerTridProviderModule.class,
       StackdriverModule.class,
       SystemClockModule.class,