Fix incorrect loop variable type (#27055)
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.
ctiller/grpc,ctiller/grpc,jtattermusch/grpc,vjpai/grpc,ctiller/grpc,grpc/grpc,ejona86/grpc,ctiller/grpc,ejona86/grpc,jtattermusch/grpc,vjpai/grpc,vjpai/grpc,stanley-cheung/grpc,vjpai/grpc,nicolasnoble/grpc,ejona86/grpc,grpc/grpc,vjpai/grpc,nicolasnoble/grpc,grpc/grpc,grpc/grpc,nicolasnoble/grpc,nicolasnoble/grpc,ejona86/grpc,ctiller/grpc,ctiller/grpc,ejona86/grpc,stanley-cheung/grpc,grpc/grpc,stanley-cheung/grpc,nicolasnoble/grpc,vjpai/grpc,jtattermusch/grpc,ejona86/grpc,stanley-cheung/grpc,nicolasnoble/grpc,jtattermusch/grpc,ejona86/grpc,grpc/grpc,vjpai/grpc,jtattermusch/grpc,grpc/grpc,vjpai/grpc,grpc/grpc,ejona86/grpc,vjpai/grpc,grpc/grpc,jtattermusch/grpc,ctiller/grpc,nicolasnoble/grpc,stanley-cheung/grpc,jtattermusch/grpc,stanley-cheung/grpc,stanley-cheung/grpc,ctiller/grpc,stanley-cheung/grpc,ctiller/grpc,nicolasnoble/grpc,ejona86/grpc,nicolasnoble/grpc,vjpai/grpc,ejona86/grpc,nicolasnoble/grpc,grpc/grpc,jtattermusch/grpc,grpc/grpc,nicolasnoble/grpc,jtattermusch/grpc,ejona86/grpc,stanley-cheung/grpc,vjpai/grpc,ctiller/grpc,jtattermusch/grpc,stanley-cheung/grpc,jtattermusch/grpc,nicolasnoble/grpc,ejona86/grpc,grpc/grpc,ctiller/grpc,jtattermusch/grpc,stanley-cheung/grpc,stanley-cheung/grpc,vjpai/grpc,ctiller/grpc