Add "scenario" as default system tag

[imiric]

This tags all metrics emitted in the main "exec" function with a "scenario" tag set to the scenario name or "default" if no custom scenarios were defined. It can be disabled with the --system-tags option.

There's no issue for this specific feature, but see #796 and #1300.

[na--]

LGTM! We really should refactor runFn... eventually... 😅

[codecov-commenter]

Codecov Report

Merging #1518 into new-schedulers will increase coverage by 0.05%.
The diff coverage is 100.00%.

@@                Coverage Diff                 @@
##           new-schedulers    #1518      +/-   ##
==================================================
+ Coverage           76.64%   76.69%   +0.05%     
==================================================
  Files                 163      163              
  Lines               13044    13047       +3     
==================================================
+ Hits                 9998    10007       +9     
+ Misses               2532     2527       -5     
+ Partials              514      513       -1     

Impacted Files	Coverage Δ
stats/system_tag.go	81.13% <ø> (ø)
stats/system_tag_set_gen.go	61.53% <ø> (ø)
js/runner.go	82.62% <100.00%> (+0.12%)	⬆️
lib/executor/helpers.go	96.73% <100.00%> (+2.23%)	⬆️
lib/executor/vu_handle.go	93.69% <0.00%> (-1.81%)	⬇️
stats/cloud/collector.go	78.70% <0.00%> (+0.61%)	⬆️
lib/executors.go	94.11% <0.00%> (+1.96%)	⬆️
lib/testutils/minirunner/minirunner.go	81.48% <0.00%> (+3.70%)	⬆️

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[mstoykov]

So we don't want to let people overwrite scenario as a tag ?

[na--]

hmm 😕 what's stopping them from overwriting it in the per-request tags?

[mstoykov]

we merge them above in tags and here we set tags[scenario] regardless :). Same btw goes for iter, vu and group. To be honest, I am not against us overwriting them, but maybe at least have some log about it.

Although this will probably need to be as part of #1478 as here it will be too noisy

[na--]

hmm the tags we merge with above are the VU activation tags, I think... so yes, I don't think you should be able to overwrite the k6 system tags with the per-scenario tags

[imiric]

Yeah, I also considered it a deliberate design choice, especially since there's a workaround: they can create a custom tag with the same name and use --system-tags to disable the built-in ones. So I'm OK with this approach too.

[mstoykov]

Yeah, I am also okay with us overwriting it. I would just prefer if we log it somewhere when it happens.

With #1478 I can do it once per initialization ... which is probably way too noisy still ... :(

[mstoykov]

but you can (and should) just merge this for now :D