@RestrictedApi(explanation = "Should only be called in TestMetricsTableMetricsMap", link = "",
      allowedOnPath = ".*/(DynamicMetricsRegistry|TestMetricsTableMetricsMap).java")
  public ConcurrentMap<String, MutableMetric> getMetricsMap() {
    return metricsMap;
  }
Get a MetricMutableCounterLong from the storage. If it is not there atomically put it.
@param counterName            Name of the counter to get
@param potentialStartingValue starting value if we have to create a new counter
hbase-hadoop-compat/src/main/java/org/apache/hadoop/metrics2/lib/DynamicMetricsRegistry.java