Sampling-Propagation:Export/import sampling mechanism through distributed trace tags

[marcotc]

Follows up from #2282.

This PR ensures the TraceOperation#sampling_mechanism populated in #2282 is injected and extracted during distributed tracing. This happens through the _dd.p.dm tag: e.g. sampling_mechanism=1 becomes _dd.p.dm=-1

The seemly minus sign is due to this field being historically comprised of two parts, separated by a dash. The first part is temporarily under review and might not be implemented as part of _dd.p.dm in the future, but instead as a separate field. Alas, we must still support this format, as this format is understood by other tracer languages already deployed in production environments.