feat(gax): Add error attributes to golden signal metrics. #609

Job	Run time
filter	1m 11s
build (17)	13m 23s
build(8) except for gapic-generator-java	3m 50s
build(8) for gapic-generator-java	10m 51s
build(8) for showcase	9m 54s
build(21) except self-service clients	12m 17s
build (11)	13m 33s
dependency_analyzer_unit_test	1m 25s
build(25) except self-service clients	12m 24s
showcase-clirr	5m 23s
showcase (17)	10m 35s
showcase (21)	10m 33s
gapic-generator-java-bom	2m 17s
showcase (11)	10m 47s
unmanaged_dependency_check	6m 56s
showcase (25)	10m 40s
	2h 15m 59s

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