Sampled means send span data to Zipkin (or something else compatible with its data). It is a
consistent decision for an entire request (trace-scoped). For example, the value should not
move from true to false, even if the decision itself can be deferred.
Here are the valid options:
- True means the trace is reported, starting with the first span to set the value true
- False means the trace should not be reported
- Null means the decision should be deferred to the next hop
Once set to true or false, it is expected that this decision is propagated and honored
downstream.
Note: sampling does not imply the trace is invisible to others. For example, a common
practice is to generate and propagate identifiers always. This allows other systems, such as
logging, to correlate even when the tracing system has no data.