message: test: fix typo, replace `counter` with `gauge`
  Actually we want to use `gauge` here.
diff: @@ -39,7 +39,7 @@ TEST_P(UdpStatsdSinkTest, InitWithIpAddress) {
  sink.flushCounter(counter, 1);
  NiceMock<MockGauge> gauge;
  - counter.name_ = "test_gauge";
  + gauge.name_ = "test_gauge";
  sink.flushGauge(gauge, 1);
  NiceMock<MockHistogram> timer;
  