Fix incorrect loop variable type (#27055)

The iterator returns a newly-constructed `std::pair<absl::string_view, absl::string_view>`.
Compiled with `-Wrange-loop-bind-reference` on iOS due to new dependencies for
iOS tests.

test/core/util/test_lb_policies.cc

@@ -83,7 +83,7 @@ class ForwardingLoadBalancingPolicy : public LoadBalancingPolicy {
 MetadataVector CopyMetadataToVector(
     LoadBalancingPolicy::MetadataInterface* metadata) {
   MetadataVector result;
-  for (const auto& p : *metadata) {
+  for (auto p : *metadata) {
     result.push_back({std::string(p.first), std::string(p.second)});
   }
   return result;