@Override
    public void postWALWrite(ObserverContext<? extends WALCoprocessorEnvironment> ctx,
      RegionInfo info, WALKey logKey, WALEdit logEdit) throws IOException {
      walEditsCount.increment();
    }
WALObserver that has a Counter for walEdits written.
hbase-server/src/test/java/org/apache/hadoop/hbase/coprocessor/TestCoprocessorMetrics.java