code: public GobblinMetrics putIfAbsent(String id, GobblinMetrics gobblinMetrics) {
      return this.metricsCache.asMap().putIfAbsent(id, gobblinMetrics);
    }
docs: Associate a {@link GobblinMetrics} instance with a given ID if the ID is
  not already associated with a {@link GobblinMetrics} instance.
  
  @param id the given {@link GobblinMetrics} ID
  @param gobblinMetrics the {@link GobblinMetrics} instance to be associated with the given ID
  @return the previous {@link GobblinMetrics} instance associated with the ID or {@code null}
  if there's no previous {@link GobblinMetrics} instance associated with the ID