public long getAgeOfLastShippedOp(String walGroup) {
    return this.ageOfLastShippedOp.get(walGroup) == null ? 0 : ageOfLastShippedOp.get(walGroup);
  }
get age of last shipped op of given wal group. If the walGroup is null, return 0
@param walGroup which group we are getting
hbase-server/src/main/java/org/apache/hadoop/hbase/replication/regionserver/MetricsSource.java