Add AlwaysParentSample sampler

[amrmahdi]

Per the ALWAYS_PARENT sampler from the spec https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/sdk-tracing.md#built-in-samplers

[rghetia]

What should be the behavior if this is a root span? It is more of an issue in the spec since it doesn't define this. If the desired behavior for root is

• always sample then it is same as probabilitySampler with probability of 1.
• if do not sample then it is same as probabilitySampler with probability of 0.
• if sample as certain probability x then it is same as probabilitySampler with x.

Essentially it is same as probabilitySampler.

The other thing is that AlwaysParentSampler is same as ProbabilitySampler with probability of 1. If there is a parent and then simply follow the decision on the parent. That is same as AlwaysParentSample (if we clarify what should be the behavior for root span for AlwaysParentSampler).

[amrmahdi]

What should be the behavior if this is a root span? It is more of an issue in the spec since it doesn't define this. If the desired behavior for root is

• always sample then it is same as probabilitySampler with probability of 1.
• if do not sample then it is same as probabilitySampler with probability of 0.
• if sample as certain probability x then it is same as probabilitySampler with x.

Essentially it is same as probabilitySampler.

The other thing is that AlwaysParentSampler is same as ProbabilitySampler with probability of 1. If there is a parent and then simply follow the decision on the parent. That is same as AlwaysParentSample (if we clarify what should be the behavior for root span for AlwaysParentSampler).

This is a good point. I believe it should be equivalent to ProbabilitySampler with 0. And then we can just wrap the ProbabilitySampler with 0 inside the AlwaysParentSampler and clarify it in the docs.

[rghetia]

This is a good point. I believe it should be equivalent to ProbabilitySampler with 0. And then we can just wrap the ProbabilitySampler with 0 inside the AlwaysParentSampler and clarify it in the docs.

I agree on ProbabilitySampler with 0.
Actually, spec implicitly implies that on root span do not sample. From spec Returns RECORD_AND_SAMPLED if SampledFlag is set to true on parent SpanContext and NOT_RECORD otherwise.