Defining thresholds in setup

[yorugac]

Feature Description

Currently thresholds can be defined only in init context, as part of options const object. This has its limitations: if threshold must depend on a value unavailable in init context, it cannot be defined.

Such a value could be:

1. from Execution API
2. coming from an extension's functionality

In many cases, one can try to add workarounds: e.g. passing some additional env vars from the outside, etc. But it would be better and have greater readability to have definition of threshold happen in setup instead with something like:

import fancy from 'k6/x/fancy';

export const options = {
  thresholds: {
    ...
  },
};

export function setup() {
  let timeoutSecs = fancy.estimate();
  options.setThreshold({
      'vu_setup': [{
            threshold: `count==${exec.instance.vusInitialized}`,
            abortOnFail: true,
            delayAbortEval: `${timeoutSecs}s`,
       }]
  });
}
...

Suggested Solution (optional)

No response

Already existing or connected issues / PRs (optional)

Somewhat related: #785

[codebien]

If we find a way of defining a good API then probably we don't need to limit this to setup. For sure, it is required for the first iteration, but after #1321 has been implemented then we might be able to set thresholds more dynamically.