public Set<String> getMetricNames() {
    return metricNames;
  }
Cache contents represent metrics for an App / Cluster.
This is designed to work on the premise that a client makes same requests
over and over, so this caching strategy allows us to send a http request
for multiple metrics in the same query and ensure parallelization on the
metrics backend side as well.
ambari-server/src/main/java/org/apache/ambari/server/controller/metrics/timeline/cache/TimelineAppMetricCacheKey.java