feat(gax): Add error attributes to golden signal metrics. #568

Job	Run time
filter	1m 2s
showcase-clirr	5m 36s
build (17)	13m 14s
build(8) except for gapic-generator-java	3m 54s
unmanaged_dependency_check	6m 52s
build(21) except self-service clients	12m 6s
build(8) for gapic-generator-java	11m 2s
build(25) except self-service clients	12m 14s
gapic-generator-java-bom	2m 10s
build(8) for showcase	2m 37s
showcase (17)	5m 54s
dependency_analyzer_unit_test	1m 19s
build (11)	13m 18s
showcase (11)	5m 52s
showcase (21)	5m 40s
showcase (25)	5m 38s
	1h 48m 28s

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