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
3f6585fccc