code: private Map<SessionStatus, MetricsTimeVaryingInt>
    createSessionStatusToMetricsMap() {
      Map<SessionStatus, MetricsTimeVaryingInt> m =
        new HashMap<SessionStatus, MetricsTimeVaryingInt>();
      for (SessionStatus endState : SESSION_END_STATES) {
        String name = endState.toString().toLowerCase() + "_sessions";
        m.put(endState, new MetricsTimeVaryingInt(name, registry));
      }
      return m;
    }
docs: Create a map of session status -> metrics.
  @return the map.