feat(gengapic): expose telemetry attributes using gax callctx helper#1706

quartzmo wants to merge 2 commits intogoogleapis:maingoogleapis/gapic-generator-go:mainfrom

quartzmo:gengapic-telemetry-contextquartzmo/gapic-generator-go:gengapic-telemetry-contextCopy head branch name to clipboard