Allow a user to be able to add additional labels to a silence via alertmanager ui

[Kellel]

It would be good to be able to specify labels that are not part of a currently firing alert so that we can reduce the number of silences that are needed.

For exampe I would like to do the following:
{ "node": "foo", "instance": "foo" }

on an alert where only instance is firing

[prymitive]

I'm not sure how is a more fine grained silence reducing the number of silences needed, it should require you to provide more silences in the end? Isn't it the other way around? If you want to silence the whole alert group you just need to un-check instance/node specific labels in the silence form, right?
Anyway a more feature rich silence form is likely to happen, but that's one of the "20% of features that take 80% of the time" deals.

Update: I think I get it - when there are 2 instances of an alertname: foo right now you can only silence both individually, skipping instance labels will silence entire group of alerts, not just for those 2 instances, but for all new instances too.
We could pull a list of all labels from the group and allow to select multiple values, or just let the user input whatever is needed. Will experiment with it.

[prymitive]

See if #70 solves your request