public String getSpec() {
    return spec;
  }
Actual http request Uri, this does not contribute to the key behavior,
it is used solely for interoperability between @AMSPropertyProvider.MetricsRequest
and the Cache.
@return Request Uri
ambari-server/src/main/java/org/apache/ambari/server/controller/metrics/timeline/cache/TimelineAppMetricCacheKey.java