message: assert if a TLStat is registered twice
  Summary:
  I don't see any way a stat could be registered twice in the code,
  which means we should assert on it. :)
diff: @@ -509,7 +509,9 @@ template <class LockTraits>
  void ThreadLocalStatsT<LockTraits>::registerStat(TLStatT<LockTraits>* stat) {
  MainGuard g(lock_);
  - tlStats_.insert(stat);
  + auto inserted = tlStats_.insert(stat).second;
  + CHECK(inserted) << "attempted to register a stat twice: " << stat->name()
  + << " (" << tlStats_.size() << " registered)";
  }
  template <class LockTraits>
  