Support undecided observability state

[carterkozak]

The default sampler should not cause a local service to observe operations, but it should also not prevent upstream tracing-aware services from recording trace data based on their local sampler configuration. This requires samplers to support an additional undecided state in addition to observed, and not observed.

In practice this should prevent the X-B3-Sampled from being set on outgoing requests.

[schlosna]

Is this resolved by #166?

[carterkozak]

Not quite yet, I think we still want to update the sampler API to produce UNDECIDED unless specifically configured. This way requests from non-configurable clients sampled at a higher rate by reconfiguring the target server. Thoughts?

[iamdanfox]

I started implementing this, but couldn't quite figure out how a TraceSampler would ever choose between UNDECIDED and DO_NOT_SAMPLE, so put together this approach: #170

[carterkozak]

I think “undecided” should be used as the default sampler (would not impact WC services)