@Override
    public void start(CoprocessorEnvironment env) throws IOException {
      if (env instanceof RegionCoprocessorEnvironment) {
        MetricRegistry registry =
          ((RegionCoprocessorEnvironment) env).getMetricRegistryForRegionServer();

        if (preGetCounter == null) {
          preGetCounter = registry.counter("preGetRequests");
        }
      }
    }
RegionObserver that has a Counter for preGet()
hbase-server/src/test/java/org/apache/hadoop/hbase/coprocessor/TestCoprocessorMetrics.java