Emit vus and vus_max metrics per scenario

[imiric]

Some users are expecting thresholds for vus and vus_max metrics to work defined on specific scenarios, e.g. 'vus{scenario:example}': ['value>30'].

This is currently not possible since those metrics are emitted at the engine level: https://github.com/loadimpact/k6/blob/586cd110358a55f5dff3c1dc88c1980e8f71a7ea/core/engine.go#L340-L362

Feature Description

vus and vus_max should be emitted as part of executors and tagged the same way as all other metrics.

[Blinkuu]

This issue is becoming more important now, as the @grafana/k6-frontend team introduced new per-scenario overviews to the k6 Grafana Plugin.

The conversation around this has also popped up a couple of times recently. First, I found the per-scenario overview to be a bit confusing, as we always plot the VU metric, regardless of the scenario we examine in the k6 Grafana Plugin. Second, the issue was also brought up by @ka3de during his WOLT presentation.

[na--]

To be clear, at this point we probably can't change this behavior without it being a major breaking change. Besides, since vus and vus_max are Gauges, emitting wildly different values just with a different scenario tag will have a very weird behavior in the current k6... 😞

Whenever we try to tackle this, we should probably do it by adding a new metric, e.g. scenario_vus or something like that. How that is going to be handled in the end-of-test summary or with distributed execution (#140) is still a bit of a unknown 😅