Fix fuzzer bug b/309756937 (#35279)

Built on #35278, which should be landed first

Always fail parsing when `grpclb_client_stats` is included in headers -- it's a meaningless value and the only reason to include it would be some sort of attack.

Closes #35279

COPYBARA_INTEGRATE_REVIEW=https://github.com/grpc/grpc/pull/35279 from ctiller:fuzz-309756937 545448c4de
PiperOrigin-RevId: 590745978

src/core/lib/transport/metadata_batch.h

@@ -410,7 +410,8 @@ struct GrpcLbClientStatsMetadata {
   static const char* DisplayMemento(MementoType) {
     return "<internal-lb-stats>";
   }
-  static MementoType ParseMemento(Slice, bool, MetadataParseErrorFn) {
+  static MementoType ParseMemento(Slice, bool, MetadataParseErrorFn error) {
+    error("not a valid value for grpclb_client_stats", Slice());
     return nullptr;
   }
 };