code: public Optional<GobblinMetrics> get(String id) {
      return Optional.fromNullable(this.metricsCache.getIfPresent(id));
    }
docs: Get the {@link GobblinMetrics} instance associated with a given ID.
  
  @param id the given {@link GobblinMetrics} ID
  @return the {@link GobblinMetrics} instance associated with the ID, wrapped in an {@link Optional} or
  {@link Optional#absent()} if no {@link GobblinMetrics} instance for the given ID is found