trace: fix build
trace: fix build

Change-Id: I97e1ed61bee76280da8649c6b6c250ca55698a4e
Reviewed-on: https://code-review.googlesource.com/8274
Reviewed-by: Ross Light <dfccc06f14414bff5a59be7fc90abf4404b47fc8@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,michael-grosshans/google-cloud-go,michael-grosshans/google-cloud-go,googleapis/google-cloud-go