trace: guard span labels
trace: guard span labels

During finish, constructTrace also mutates the span
to add HTTP labels if necessary. Concurrent use of
SetLabel and Finish leads to a data race.

Fixes #584.

Change-Id: Iec49f183298fe7fd65e66020b1efdb978ac31d36
Reviewed-on: https://code-review.googlesource.com/11631
Reviewed-by: Hana Kim <ea6b243106897f1a7515c56fa7b92afaf1f2b2b2@google.com>

googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go,googleapis/google-cloud-go