Rename ProbabilitySampler to TraceIdRatioBasedSampler and implement ratio sampling

[dyladan]

The probability sampler should be deterministic depending on the Trace ID.

https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/trace/sdk.md#requirements-for-traceidratiobased-sampler-algorithm

[dyladan]

Assigning this to @legendecas because he is already working on the parent sampled flag in the probability sampler and this issue is related.

[naseemkullah]

do we need to rename OTEL_SAMPLING_PROBABILITY env var to OTEL_TRACE_ID_RATIO?

[dyladan]

There is no spec for it, so I guess we can do what we want: https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/sdk-environment-variables.md

I think probability is more clear to end users not familiar with our internals, and I think it is still accurate. Probability does not imply randomness.

[obecny]

@legendecas hey jsut wanted to ask what is the current status of this ticket, thx

[legendecas]

@obecny I filed a PR #1562, please take a look at your convenience :)