Support thresholds and the end-of-test summary in distributed execution

[na--]

What?

This PR is another sliver of the newly refactored #2816. It builds on top of #3205 to add very basic support for metrics (thresholds and the end-of-test summary, incl. handleSummary()) for distributed execution.

As you can see from all of the TODOs and the lack of tests, this is even more of a proof-of-concept PR than even #3205 😅 It is very far from ready for any kind of actual production usage!

Even if we ignore the super-inefficient string-y and JSON encoding of metrics data and their transmission over the wire, k6 still doesn't have HDR histograms (#763). So the central k6 coordinator node will suffer from the same memory and data processing problems k6 currently does, it will require tons of memory and CPU for crunching the metrics data of large or long-running tests...

Why?

Same as the other PRs in this series, I decided to create this PR even when the feature isn't production-ready because I think merging it this way will make it easier to build the final end result in smaller and faster iterations.

Once again, this PR is completely backwards compatible! 🎉 It doesn't have any tests, but it also doesn't disable or change any existing tests or k6 behavior. It only affects distributed execution which, if #3205 is merged, would be kind of hidden and officially quite experimental. But it mostly works and it doesn't need HDR histograms to be figured out before it can be merged! 🎉

So it makes sense to me to have this basic framework merged first and then work on HDR histograms (and the other features that would make distributed execution production-ready) in separate PRs.

Checklist

• I have performed a self-review of my code.
• I have added tests for my changes.
• I have run linter locally (make ci-like-lint) and all checks pass.
• I have run tests locally (make tests) and all tests pass.
• I have commented on my code, particularly in hard-to-understand areas.

Related PR(s)/Issue(s)

• Clustering and distributed execution #140
• Distributed Execution Proposal #3218
• Add a design doc for distributed execution and test suites #3217
• [WIP] Proof of concept changes (test suites, HDR histograms) #2816
• Use HDR histograms for calculating percentiles in thresholds and summary stats #763

[codecov-commenter]

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