feat: MetricsTracer implementation #1221

Job	Run time
downstream-compatibility (google-cloud-java)	9m 58s
downstream-compatibility (java-bigtable)	4m 4s
downstream-compatibility (java-bigquery)	3m 14s
downstream-compatibility (java-bigquerystorage)	9m 7s
downstream-compatibility (java-datastore)	2m 54s
downstream-compatibility (java-firestore)	1m 39s
downstream-compatibility (java-logging)	1m 38s
downstream-compatibility (java-logging-logback)	1m 30s
downstream-compatibility (java-pubsub)	2m 2s
downstream-compatibility (java-pubsublite)	2m 15s
downstream-compatibility (java-spanner-jdbc)	2m 17s
downstream-compatibility (java-spanner)	4m 16s
downstream-compatibility (java-storage)	3m 43s
downstream-compatibility (java-storage-nio)	1m 59s
	50m 36s

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