Imagestore: Fallback to application default credentials when no key file is specified for GCS (#25948)

The external image storage for GCS creates the JWT Token from a credentials file, 
but if your Grafana server runs under a GCE instance with a service account on it, 
you can use that instead (you don't have to manage/secure the credentials file).

Co-authored-by: Arve Knudsen <arve.knudsen@gmail.com>
Co-authored-by: Emil Tullstedt <emil.tullstedt@grafana.com>

docs/sources/administration/configuration.md

@@ -1179,7 +1179,7 @@ Optional URL to send to users in notifications. If the string contains the seque
 
 ### key_file
 
-Path to JSON key file associated with a Google service account to authenticate and authorize.
+Optional path to JSON key file associated with a Google service account to authenticate and authorize. If no value is provided it tries to use the [application default credentials](https://cloud.google.com/docs/authentication/production#finding_credentials_automatically).
 Service Account keys can be created and downloaded from https://console.developers.google.com/permissions/serviceaccounts.
 
 Service Account should have "Storage Object Writer" role. The access control model of the bucket needs to be "Set object-level and bucket-level permissions". Grafana itself will make the images public readable.

pkg/components/imguploader/gcsuploader.go

@@ -43,20 +43,31 @@ func (u *GCSUploader) Upload(ctx context.Context, imageDiskPath string) (string,
 	fileName += pngExt
 	key := path.Join(u.path, fileName)
 
-	u.log.Debug("Opening key file ", u.keyFile)
-	data, err := ioutil.ReadFile(u.keyFile)
-	if err != nil {
-		return "", err
-	}
-
-	u.log.Debug("Creating JWT conf")
-	conf, err := google.JWTConfigFromJSON(data, tokenUrl)
-	if err != nil {
-		return "", err
+	var client *http.Client
+
+	if u.keyFile != "" {
+		u.log.Debug("Opening key file ", u.keyFile)
+		data, err := ioutil.ReadFile(u.keyFile)
+		if err != nil {
+			return "", err
+		}
+
+		u.log.Debug("Creating JWT conf")
+		conf, err := google.JWTConfigFromJSON(data, tokenUrl)
+		if err != nil {
+			return "", err
+		}
+
+		u.log.Debug("Creating HTTP client")
+		client = conf.Client(ctx)
+	} else {
+		u.log.Debug("Key file is empty, trying to use application default credentials")
+		client, err = google.DefaultClient(ctx)
+		if err != nil {
+			return "", err
+		}
 	}
 
-	u.log.Debug("Creating HTTP client")
-	client := conf.Client(ctx)
 	err = u.uploadFile(client, imageDiskPath, key)
 	if err != nil {
 		return "", err