Minor cleanup and fixes

- Do not allow '.' as path component, because it undermines depth
  checks, and add tests
- Fix GiB reporting
- Fix metrics label
- Helper function for http errors

metrics.go

@@ -63,7 +63,7 @@ func makeBlobMetricFunc(username string, folderPath []string) repo.BlobMetricFun
 	var f repo.BlobMetricFunc = func(objectType string, operation repo.BlobOperation, nBytes uint64) {
 		labels := prometheus.Labels{
 			"user": username,
-			"repo": strings.Join(folderPath, ""),
+			"repo": strings.Join(folderPath, "/"),
 			"type": objectType,
 		}
 		switch operation {