perf(gax-grpc): skip metadata materialization when logging is disabled #17742

Job	Run time
root-pom	1m 43s
group_id_check_for_maps_libraries	8m 13s
owlbot-py	1m 12s
owlbot-yaml	1m 21s
package_name_check	1m 14s
gapic-bom	1m 47s
gitignore_correctness	1m 5s
generate-readme	1m 7s
	17m 42s

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