Introduce Stackdriver Trace support

This patch implements support for Stackdriver Trace by means of OpenCensus. It takes quite a bit of wrangling to get it to play nicely with OpenTracing. Eventually we may be able to throw all of this out in favour of OpenTelemetry, hopefully.

But this seems like an acceptable middle ground. The goal is to get tracing into gitaly ASAP https://gitlab.com/gitlab-com/gl-infra/infrastructure/-/issues/13622. With a longer-term plan of moving to Jaeger gitlab-com/gl-infra&210 (closed).

I'm focusing on labkit-go only for now. Ruby is considered out of scope.

It's been tested successfully with gdk, with the feature compiled into gitaly and enabled on gitaly and praefect.