Merge with other metadata changes

src/core/transport/metadata.c

@@ -561,9 +561,9 @@ void grpc_mdctx_lock(grpc_mdctx *ctx) { lock(ctx); }
 void grpc_mdctx_locked_mdelem_unref(grpc_mdctx *ctx, grpc_mdelem *gmd) {
   internal_metadata *md = (internal_metadata *)gmd;
   grpc_mdctx *elem_ctx = md->context;
-  GPR_ASSERT(md->refs);
   GPR_ASSERT(ctx == elem_ctx);
-  if (0 == --md->refs) {
+  assert(gpr_atm_no_barrier_load(&md->refcnt) >= 1);
+  if (1 == gpr_atm_full_fetch_add(&md->refcnt, -1)) {
     ctx->mdtab_free++;
   }
 }