Per-group alert acknowledgment for grouped alerts #2599

For grouped alerts, add a way to acknowledge or silence a single alert group without muting the entire parent alert.

Right now, acknowledging an alert mutes the whole alert. For alerts that evaluate multiple groups, that is too coarse, already-investigated group needs to be silenced temporarily while other groups should continue to notify normally.

Desired behavior

When an alert is grouped, users should be able to acknowledge a specific group only.

That per-group acknowledgment should:

• suppress notifications only for the acknowledged group
• leave other groups in the same alert unaffected
• preserve normal alert evaluation for all groups

Whole-alert acknowledgment should continue to exist and should remain the "ack all groups" behavior.