Add alwaysRecord Sampler

[blueswhisper]

Is your feature request related to a problem? Please describe.
Add Sampler.alwaysRecord() sampler return SamplingDecision.RECORD_ONLY decision to build real Span always. But leave the decision whether real export span data on span end to implement some error span force sample logic.

Most normal success span will just drop on span end. Only error status span and sampled span export to trace system for diagnostic. Error span has higher possiblity for user query.

Describe alternatives you've considered
Config sample rate to 100%, leave sample decision to opentelemetry-collector. May cause high pressure for opentelemetry-collector for high deploy density K8s Node.

[jkwatson]

If we want to provide this Sampler as a part of the core SDK, we'd need to have it specified in the official specification first. In the mean time, you can definitely implement this yourself where you want to use it, correct?

[blueswhisper]

Yes. I have implemented my own alwaysRecord Sampler actually.

[jack-berg]

Implementation proposed in #7877