Add option for X-Ray remote sampling

[gillesbergerp]

We have successfully integrated OpenTelemetry with various of our Java applications (with X-Ray as a backend). Luckily, the OpenTelemetry Java SDK supports an X-Ray remote sampler that periodically polls X-Ray for sampling rules and applies them based on resource attributes (service name/type/...).
It would be great if the Ruby SDK would add support for this functionality as well since it makes managing sampling across a fleet of applications much easier.

[mwear]

Thanks for opening this issue. We've moved it to the contrib repo. Is there any chance you'd be interested in contributing this functionality?

[gillesbergerp]

Definitely. I just don't know how much time I can dedicate to this. I'd also need a little guidance as to where it would best fit in. Should we start a new sampling package similar to instrumentaion and propagator?

[arielvalentin]

@gillesbergerp yes, I think that creating a new sampling directory makes sense.

I think each sampler should be encapsulated in its own gem.

[gillesbergerp]

I agree, that makes total sense. I'll see if I can find some time on the weekend and will reach out if I need anything.

[gillesbergerp]

X-Ray falls back to "1r/s or 5% after that" sampling rule if it cannot find any matching remote rules. I thought about making that implicit in the X-Ray sampling gem but then came to the conclusion that this could be interesting as a general option as well. At the very least, it will keep the X-Ray sampling implementation less complex. What do you think?
See #717

[gillesbergerp]

Please see #719 for the X-Ray gem. We'll also need adaptions in https://github.com/open-telemetry/opentelemetry-ruby (specifically here) to allow for initializing the new sampler.

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