// 500 Internal Server Error.
type BucketMetricsLoaderFn func(context.Context, MetricValues, *metricsCache, []string) error

// JoinBucketLoaders - joins multiple bucket loaders into a single loader,
// similar to `JoinLoaders`.
func JoinBucketLoaders(loaders ...BucketMetricsLoaderFn) BucketMetricsLoaderFn {
	return func(ctx context.Context, m MetricValues, c *metricsCache, b []string) error {

			bucketAPIRequestsTotalMD,
			bucketAPIRequestsCanceledMD,
			bucketAPIRequests4xxErrorsMD,
			bucketAPIRequests5xxErrorsMD,

			bucketAPIRequestsTTFBSecondsDistributionMD,
		},
		JoinBucketLoaders(loadBucketAPIHTTPMetrics, loadBucketAPITTFBMetrics),
	)

	bucketReplicationMG := NewBucketMetricsGroup(bucketReplicationCollectorPath,
		[]MetricDescriptor{
			bucketReplLastHrFailedBytesMD,