With Java 8 in GAE standard environment, we can now use standard java thread factory to run the metric reporter in the background in daemon mode, which would not interfere with basic scaling idle timeout as App Engine thread would.
Because the thread is not created by ThreadManager, no App Engine APIs can be called from it. We therefore use GoogleCredential instead of AppIdentityCredential as HttpRequestInitializer, and NetHttpTransport instead of UlrFetchTransport as HttpTransport.
MetricReporter is lazy injected because it depends on jsonCredential retrieved from CloudKms, which is not available in a test environment, causing FrontendServletTest and BackendServletTest to fail.
Some minor re-formatting with google-java-format on edited files.
Lastly removed moe comments in import statement, which makes the linter unhappy.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=172896227
c702b4486c