code: private final RequestMetricCollector findRequestMetricCollector(
              RequestMetricCollector reqLevelMetricsCollector) {
          if (reqLevelMetricsCollector != null) {
              return reqLevelMetricsCollector;
          } else if (getRequestMetricsCollector() != null) {
              return getRequestMetricsCollector();
          } else {
              return AwsSdkMetrics.getRequestMetricCollector();
          }
      }
docs: Returns the most specific request metric collector, starting from the request level, then
  client level, then finally the AWS SDK level.