Add sampling capabilities to the tracing module

[oleiade]

About

This Pull Request adds probabilistic sampling support to the tracing module. Using it, users can specify the percentage of their instrumented requests which should have their trace context header's sampled field set to true.

From a user-interface perspective, this feature is introduced as an option meant to be set during either the instantiation of a tracing.Client, or during a call to instrumentHTTP. It consists of a single integer field of the tracing options, whose value, within the 0 <= n <= 100 bounds, is meant to represent a percentage.

// The sampling option is available when instantiating a Client
const sampledHTTP = new tracing.Client({
	propagator: "w3c",
	
	// Up to 55% of the requests emitted using the client instance's
	// methods will have their trace context header's sampled field
	// set to true.
	sampling: 55,
})

// It is also available when using the instrumentHTTP function
tracing.InstrumentHTTP({
	propagator: "w3c",
	
	// Up to 27% of the requests made with the HTTP module's functions
	// will have their trace context header's sampled field set to true
	// from this point onward 
	sampling: 27,
})

This feature is implemented using probabilistic sampling. Meaning that over an extensive series of iterations, the number of requests with their sampled flag set to true will tend to converge to the selected sampling rate value. It was a design choice without explicit constraints, preferring randomness over accuracy: each trace context generation evaluates its chance of being sampled independently. If you find an implementation emphasizing accuracy more suitable, I'm open to exploring that route; just let me know.

Note that regardless of the sampled header flag's value, the trace_id metadata will be attached to each output sample.

[codecov-commenter]

Codecov Report

Merging #2886 (5cd1416) into master (37cd20b) will increase coverage by 0.11%.
The diff coverage is 82.05%.

❗ Current head 5cd1416 differs from pull request most recent head 1e9926b. Consider uploading reports for the commit 1e9926b to get more accurate results

@@            Coverage Diff             @@
##           master    #2886      +/-   ##
==========================================
+ Coverage   76.83%   76.94%   +0.11%     
==========================================
  Files         229      230       +1     
  Lines       16923    16978      +55     
==========================================
+ Hits        13002    13063      +61     
+ Misses       3078     3076       -2     
+ Partials      843      839       -4     

Flag	Coverage Δ
ubuntu	76.85% <82.05%> (+0.08%)	⬆️
windows	76.76% <82.05%> (+0.12%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
cmd/tests/tests.go	46.15% <0.00%> (+1.70%)	⬆️
log/loki.go	30.70% <0.00%> (-6.36%)	⬇️
js/modules/k6/experimental/tracing/encoding.go	66.66% <40.00%> (-33.34%)	⬇️
js/modules/k6/experimental/tracing/sampling.go	50.00% <50.00%> (ø)
log/file.go	79.66% <50.00%> (-1.59%)	⬇️
js/modules/k6/experimental/tracing/module.go	71.69% <66.66%> (+0.54%)	⬆️
js/modules/k6/experimental/tracing/options.go	88.88% <77.77%> (+25.25%)	⬆️
lib/testutils/httpmultibin/httpmultibin.go	92.64% <89.18%> (+2.69%)	⬆️
cmd/root.go	95.33% <94.54%> (+13.51%)	⬆️
js/modules/k6/experimental/tracing/client.go	69.72% <100.00%> (+0.85%)	⬆️
... and 3 more

... and 3 files with indirect coverage changes

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[oleiade]

For your information, 377abf9 incorporates improvements to the assertHasTraceIDMetadata function as suggested by @mstoykov in previous PRs 👍🏻

[codebien]

It could be good to address #2889 here

[oleiade]

@codebien I'll give it shot, if it takes more than an hour or two, I would address it during cooldown in a separate PR 👍🏻

[oleiade]

@codebien #2889 was addressed in a separate PR #2894 🚀

[mstoykov]

LGTM!

A nitpick was that it seems to be we can just inline the chance function, but I am okay either way

[codebien]

Overall LGTM, has some minor comments. Really well done, I'm out from tracing domain but it has been easy to understand 👏

[oleiade]

Addressed all comments, looking forward to your 👍🏻 👏🏻 🙇🏻

[codebien]

LGTM

I restarted the failed checks