code: private void buildCustomMetricReporters(Properties properties) {
      String reporterClasses = properties.getProperty(ConfigurationKeys.METRICS_CUSTOM_BUILDERS);
  
      if (Strings.isNullOrEmpty(reporterClasses)) {
        return;
      }
  
      for (String reporterClass : Splitter.on(",").split(reporterClasses)) {
        buildScheduledReporter(properties, reporterClass, Optional.<String>absent());
      }
    }
docs: Build scheduled metrics reporters by reflection from the property
  {@link org.apache.gobblin.configuration.ConfigurationKeys#METRICS_CUSTOM_BUILDERS}. This allows users to specify custom
  reporters for Gobblin runtime without having to modify the code.