impl(o11y): introduce body.size #16936

Job	Run time
owlbot-py	1m 21s
gapic-bom	1m 32s
generate-readme	1m 21s
group_id_check_for_maps_libraries	7m 42s
gitignore_correctness	1m 17s
package_name_check	1m 16s
owlbot-yaml	1m 14s
root-pom	1m 54s
	17m 37s

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