Create Sampler API

[fractalwrench]

Goal

Creates the interface that forms the public Sampler API. Currently there is no way to configure or use these symbols - that will happen in subsequent PRs.

Closes #34

[codecov]

Codecov Report

❌ Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.
✅ Project coverage is 90.60%. Comparing base (5f111d7) to head (edda42e).
⚠️ Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
...elemetry/kotlin/tracing/sampling/SamplingResult.kt	0.00%	3 Missing ⚠️

❌ Your patch status has failed because the patch coverage (0.00%) is below the target coverage (50.00%). You can increase the patch coverage or adjust the target coverage.
❌ Your project status has failed because the head coverage (90.60%) is below the adjusted base coverage (90.71%). You can increase the head coverage or adjust the Removed Code Behavior.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #51      +/-   ##
==========================================
- Coverage   90.71%   90.60%   -0.12%     
==========================================
  Files         214      215       +1     
  Lines        2295     2298       +3     
  Branches      233      233              
==========================================
  Hits         2082     2082              
- Misses        160      163       +3     
  Partials       53       53              

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[bidetofevil]

Give how sampling should work on mobile apps without it being fragmented and biased (i.e. not per telemetry instance, but by app instance or install), it feels like this merits a bigger discussion to see if it makes sense to bake that into the API.

Thoughts?

[fractalwrench]

Happy to discuss this.

My first thoughts are that the Sampler API is a stable part of the OpenTelemetry specification and that this project also needs to support backend use-cases, not just mobile. IMO the Sampler API is fairly extensible and it seems possible to write a custom sampler that takes app instance/install ID into account.

[bidetofevil]

LGTM. I guess my main issue is whether the base API is sufficiently expressive and what Sampler implementations ought to be built initially. Both of these could be done after the core API is added.