Fixup 12494: Fixes for logging and additional comment

Author: sauravzg

xds/src/main/java/io/grpc/xds/internal/headermutations/HeaderMutationFilter.java

@@ -33,7 +33,7 @@ public class HeaderMutationFilter {
 
 
 
-  public HeaderMutationFilter(Optional<HeaderMutationRulesConfig> mutationRules) { // NOPMD
+  public HeaderMutationFilter(Optional<HeaderMutationRulesConfig> mutationRules) {
     this.mutationRules = mutationRules;
   }
 

xds/src/main/java/io/grpc/xds/internal/headermutations/HeaderMutator.java

@@ -111,7 +111,7 @@ private <T> void updateHeader(final HeaderAppendAction action, final Metadata.Ke
 
       default:
         // Should be unreachable unless there's a proto schema mismatch.
-        logger.warning("Unknown HeaderAppendAction: " + action);
+        logger.fine("Unknown HeaderAppendAction: " + action);
     }
 
     if (!keepEmptyValue) {