Add the possibility to add a custom label to ALL metrics generated

[giorgiodig]

We are running playwright tests for multiple environments (dev, stage, prod), and it would be very nice to add a custom label support in playwright.config.js, for ALL metrics generated by playwright, so we can differentiate between environments when building dashboards, alerts, etc.

Right now we are doing a non-ideal thing of creating different prefixes based on the environment, but it is not ideal for us as it:

• generates duplicated metrics
• makes dashboarding harder, as we can't filter by label name, but we have to hardcode metrics name (and prefixes)

Thanks!

[xTrekStorex]

see #9 & #10

[vitalics]

close as duplicate of #9