Adjust buckets for blob sizes

The blob size buckets for prometheus metrics were set up in the same
way as the time buckets, however this didn't make sense as it resulted
in buckets for < 1 byte which is impossible

They have been adjusted to match the buckets used in
bb-storage/blobstore

Author: sdclarke

pkg/fetch/metrics_fetcher.go

@@ -23,7 +23,7 @@ var (
 			Subsystem: "remote_asset",
 			Name:      "http_fetcher_blob_size_bytes",
 			Help:      "Size of blobs fetched using the http fetcher, in bytes",
-			Buckets:   util.DecimalExponentialBuckets(1, 6, 2),
+			Buckets:   prometheus.ExponentialBuckets(1.0, 2.0, 33),
 		},
 		[]string{"name", "operation", "resource_type"})
 	blobAccessOperationsDurationSeconds = prometheus.NewHistogramVec(

pkg/push/metrics_push_server.go

@@ -22,7 +22,7 @@ var (
 			Subsystem: "push_server",
 			Name:      "push_server_blob_size_bytes",
 			Help:      "Size of blobs being pushed, in bytes.",
-			Buckets:   util.DecimalExponentialBuckets(1, 6, 2),
+			Buckets:   prometheus.ExponentialBuckets(1.0, 2.0, 33),
 		},
 		[]string{"name", "operation", "resource_type"})
 	pushServerOperationsDurationSeconds = prometheus.NewHistogramVec(