impl(o11y): introduce body.size #210

Job	Run time
filter	1m 6s
downstream-compatibility-spring-generator	2m 31s
downstream-compatibility (java-firestore)	2m 4s
downstream-compatibility (java-bigtable)	2m 0s
downstream-compatibility (java-spanner-jdbc)	2m 24s
downstream-compatibility (java-pubsublite)	2m 11s
downstream-compatibility (java-storage)	2m 21s
downstream-compatibility (java-bigquery)	2m 3s
downstream-compatibility (java-storage-nio)	2m 9s
downstream-compatibility (java-pubsub)	2m 21s
downstream-compatibility (java-spanner)	2m 14s
	23m 24s

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