Add example for sub-metrics by scenario (or other tags) in summary

While we're waiting for the proper solution that would allow users to explicitly track certain sub-metrics (https://github.com/loadimpact/k6/issues/1321), we should probably document with an example the current workaround for it with bogus thresholds. I've had to mention it multiple times in the forum, so a central generic page in the docs we can link to would be useful:
- https://community.k6.io/t/multiple-scenarios-metrics-per-each/1314/3
- https://community.k6.io/t/ignore-http-calls-made-in-setup-or-teardown-in-results/878/2
- https://community.k6.io/t/separate-metrics-summary-for-each-request-in-default-function/1223/2
- https://github.com/loadimpact/k6/issues/1836#issuecomment-771576459

Also, we should mention more visibly that scenarios tag metrics with the `scenario: name` tag. We currently mention it off-hand [below this "advanced" examples](https://k6.io/docs/using-k6/scenarios/advanced-examples#different-environment-variables-and-tags-per-scenario), when we probably should add it to the list on the [main Scenarios page](https://k6.io/docs/using-k6/scenarios).