Fix ProbabilitySampler

[fbogsany]

open-telemetry/opentelemetry-specification#704 simplified the probability sampler, relying on parent_or_else to respect the parent context's sampled flag, if required. This PR updates the probability sampler accordingly.

#321 highlighted that actually setting the sampler to something other than the default is cumbersome. There are 2 levels of complexity here:

1. indirection through the TracerProvider's active trace config
2. deep nesting of modules.

This also fixes 2. above by adding a few helpers for the common cases of parent_or_else delegating to one of the standard samplers. E.g.

delegate = OpenTelemetry::SDK::Trace::Samplers.probability(0.001)
OpenTelemetry::SDK::Trace::Samplers.parent_or_else(delegate)

becomes:

OpenTelemetry::SDK::Trace::Samplers.parent_or_probability(0.001)

[mwear]

LGTM