errorreporting: support user provided stacktrace
errorreporting: support user provided stacktrace

Change-Id: I770635eb4a1e88e87b6ab9f8fdd49220205b92fe
Reviewed-on: https://code-review.googlesource.com/20270
Reviewed-by: kokoro <2ac7b1f3fa578934c95181d4272be0d3bca00121@google.com>
Reviewed-by: JBD <347c19a639ce752a783b4ea325d6539ab4b4a262@google.com>

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