Implement tracing sampling status checker
This MR implements IsSampled function to extract the sampling status of a particular span. 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 (Draft: Tracing: Have a deeper look into Git pro... (gitaly!5421 - closed)). 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.