perf(gax-grpc): skip metadata materialization when logging is disabled #1017

Job	Run time
filter	1m 12s
downstream-compatibility (java-firestore)	4m 54s
downstream-compatibility-spring-generator	11m 53s
downstream-compatibility (java-pubsub)	4m 39s
downstream-compatibility (java-pubsublite)	5m 42s
downstream-compatibility (java-bigtable)	4m 13s
	32m 33s

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