fix: use version.repo in userAgent version

Context:
When putting together per version usage dashboard for GCS libraries, we found that concord has not been updated to use x-goog-api-client information instead of user-agent information.
Ref: go/gcs-client-library-usage

I'm sending this change to help understand usage of newer releases when Domain Split for GCS is introduced into the library (go/gcs-domain-split-manual-library-rollout-strategy). The change updates the userAgent value to use the value from x-goog-api-client repo version instead.

Change-Id: Ia205585f6d8575b1b81cd638c289310d1b851ca5
Reviewed-on: https://code-review.googlesource.com/c/gocloud/+/47651
Reviewed-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jean de Klerk <deklerk@google.com>
diff --git a/storage/storage.go b/storage/storage.go
index 1ffb10f..43f55ee 100644
--- a/storage/storage.go
+++ b/storage/storage.go
@@ -54,7 +54,7 @@
 	ErrObjectNotExist = errors.New("storage: object doesn't exist")
 )
 
-const userAgent = "gcloud-golang-storage/20151204"
+var userAgent = fmt.Sprintf("gcloud-golang-storage/%s", version.Repo)
 
 const (
 	// ScopeFullControl grants permissions to manage your