Most implementations support checking this status. Unfortunately, we hide the
tracing implementations behind unified opentracing interfaces. The single
input is an opentracing.Span interface. As a result, we need to type-cast it
back to all supported implementations.

This status is particularly useful when a span is not free to collect, or
has to turn on some special configs. One example is Git Trace2. It's a
built-in observability tool that provides a deeper look into Git processes.
Enabling this feature is not expensive, but not cheap either. We don't want
to enable it for all Git processes. So, it makes sense to collect such data
when the parent span is sampled.

Some implementations follow opentracing. Labkit doesn't need to convert.
Some others, such as stackdriver, implement opentracing wrappers. Those
wrappers are internal, and conditional built when receiving corresponding
build tag. As a result, such wrappers are not available for casting in
the checker. To overcome this, the context wrappers must implement a simple
samplingChecker interface.

Changelog: added
Issue: gitaly#4805