fix: make some enum fields final (#1526)

* Make some enum fields final

See go/bugpattern/ImmutableEnumChecker

* One more field

Author: cushon

credentials/java/com/google/auth/CredentialTypeForMetrics.java

@@ -52,7 +52,7 @@ public enum CredentialTypeForMetrics {
   IMPERSONATED_CREDENTIALS("imp"),
   DO_NOT_SEND("dns");
 
-  private String label;
+  private final String label;
 
   private CredentialTypeForMetrics(String label) {
     this.label = label;

oauth2_http/java/com/google/auth/oauth2/IdTokenProvider.java

@@ -59,7 +59,7 @@ public enum Option {
     LICENSES_TRUE("licensesTrue"),
     INCLUDE_EMAIL("includeEmail");
 
-    private String option;
+    private final String option;
 
     private Option(String option) {
       this.option = option;

oauth2_http/java/com/google/auth/oauth2/MetricsUtils.java

@@ -78,7 +78,7 @@ public enum RequestType {
     METADATA_SERVER_PING("mds"),
     UNTRACKED("untracked");
 
-    private String label;
+    private final String label;
 
     private RequestType(String label) {
       this.label = label;