Initialize all fields in default constructor (#27823)

This triggers MSAN reports with -Xclang -enable-noundef-analysis -mllvm -msan-eager-checks=1

src/core/lib/transport/parsed_metadata.h

@@ -80,7 +80,7 @@ class ParsedMetadata {
                     : MdelemVtable<false>()),
         value_(static_cast<intptr_t>(elem.payload)),
         transport_size_(GRPC_MDELEM_LENGTH(elem)) {}
-  ParsedMetadata() : vtable_(EmptyVTable()) {}
+  ParsedMetadata() : vtable_(EmptyVTable()), value_(0), transport_size_(0) {}
   ~ParsedMetadata() { vtable_->destroy(value_); }
 
   // Non copyable, but movable.