Added BYOID Metrics

google/internal/externalaccount/filecredsource.go

@@ -19,6 +19,10 @@ type fileCredentialSource struct {
 	Format format
 }
 
+func (cs fileCredentialSource) credentialSourceType() string {
+	return "file"
+}
+
 func (cs fileCredentialSource) subjectToken() (string, error) {
 	tokenFile, err := os.Open(cs.File)
 	if err != nil {