Re-add group key to alert groups endpoint

[Spaceman1701]

I'm working on some integrations which would benefit from associating webhook notifications with alerts returned from the results from alerts/groups. It'd be super useful if this endpoint included the group key for each group.

It looks like the old v1 api did include the group key in AlertGroup (#576) but when the new v2 api was added this field was omitted: #1791

There doesn't seem to be any discussion of the group key in #1791 so my guess is that it was just an accidental removal. Are there any concerns about re-adding this?

[simonpasquier]

I participated in the discussions you've mentioned but can't remember if the choice of not including the group key was deliberate or not 🤔
I've got no strong objection of adding the information but I'd like to hear from @gotjosh too.

[fihufil]

Hello everyone!

Reintroduction of this feature would be really welcome as it would allow us to do some more automation around Alertmanager, is it possible to reconsider this PR?

KR,
Filip Dorosz