refactor: Break samplers into classes and store them on agent.sampler.*

[amychisholm03]

Description

Fixes some sampling-related bugs by better encapsulating remote_parent_sampled and remote_parent_not_sampled sampling decisions with agent.sampler.remoteParentSampled and agent.sampler.remoteParentNotSampled. Also, replaces agent.transactionSampler with agent.sampler.root, so quite a few tests were modified to account for this.

Introduces the concept of a Sampler interface with exposes applySamplingDecision, which updates a given Transaction's priority and sampled flags.

How to Test

npm run unit
npm run integration

Related Issues

Precursor for core tracing and TraceIdRatioBasedSampler work.

[codecov]

Codecov Report

❌ Patch coverage is 94.51220% with 9 lines in your changes missing coverage. Please review.
✅ Project coverage is 89.49%. Comparing base (47ed964) to head (7d0d13b).
⚠️ Report is 3 commits behind head on main.

Files with missing lines	Patch %	Lines
lib/transaction/index.js	83.33%	5 Missing ⚠️
lib/agent.js	81.81%	2 Missing ⚠️
lib/samplers/sampler.js	90.90%	2 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3527      +/-   ##
==========================================
+ Coverage   81.48%   89.49%   +8.01%     
==========================================
  Files         411      425      +14     
  Lines       54553    55903    +1350     
  Branches        1        1              
==========================================
+ Hits        44455    50033    +5578     
+ Misses      10098     5870    -4228     

Flag	Coverage Δ
integration-tests-cjs-20.x	73.93% <93.29%> (?)
integration-tests-esm-20.x	52.65% <60.36%> (+0.06%)	⬆️
integration-tests-esm-22.x	52.69% <60.36%> (?)
versioned-tests-20.x	81.06% <80.48%> (-0.32%)	⬇️
versioned-tests-22.x	81.07% <80.48%> (?)

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[svetlanabrennan]

This hasn't been decided yet. The spec says to use both but agent.sampler.full_granularity.* takes precedence over agent.sampler.*.

[amychisholm03]

When I mean "deprecated", I'm referring to the agent.sampler.root, agent.sampler.remoteParentSampled, and agent.sampler.remoteParentNotSampled fields on the agent, not the config. In the future, it will be agent.sampler.fullGranularity.root, etc., which assumes the precedence logic of sampler.fullGranularity > sampler handled by the config initialization.

[svetlanabrennan]

Should we make these camel case since that's the preferred way?

[amychisholm03]

This is more pseudo-codey than anything else, but I can make that change

[svetlanabrennan]

So the transactionSampler is always the root sampler?

[amychisholm03]

Yes, it was.

[amychisholm03]

Though this updateSamplingTarget function does reveal another thing we will have to resolve with the sharing of AdaptiveSampler states, but that's out of scope for this PR.