The `service.name` attribute is used by the GCP Trace Explorer to allow
filtering by service. Users would reasonably expect that when they use the
`WithServiceName` option, that this attribute would be set in traces.

fix(tracing): Add the `service.name` default attribute in the Stackdriver tracer.

See merge request !234



Merged-by: Steve Xuereb <sxuereb@gitlab.com>
Approved-by: Steve Xuereb <sxuereb@gitlab.com>
Co-authored-by: Florian Forster <fforster@gitlab.com>