perf(gax-grpc): skip metadata materialization when logging is disabled #1189

Job	Run time
filter	1m 3s
build(8) for showcase	10m 0s
build(25) except self-service clients	9m 58s
build(8) for gapic-generator-java	7m 52s
dependency_analyzer_unit_test	1m 29s
build(21) except self-service clients	9m 28s
unmanaged_dependency_check	13m 35s
build (17)	9m 20s
build (11)	9m 39s
showcase (25)	14m 26s
showcase (21)	13m 36s
showcase-clirr	5m 13s
showcase (17)	13m 36s
build(8) except for gapic-generator-java	3m 49s
gapic-generator-java-bom	3m 2s
showcase (11)	13m 48s
	2h 19m 54s

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