consider the sample decision of implicit parent spans (when creating
a span without explicitly providing a context) instead of forwarding
to the delegating sampler.

* samplers did not return the trace_state in the sampling result
  when a span was dropped. This caused the trace_state extracted
  from a remote parent span to be erased and further context
  propagation to break.
* fix also TraceIdRatioBased which erased the trace_state independent
  of the sampling outcome.