[alertmanager] PR: Add threaded message strategy for Slack notifier

[Santiago Fernández Núñez]

Hey all!

I'm Santiago, working at Nubank — we're heavy Prometheus users and have been investing in the ecosystem for a while. I've put together a PR for something that's been a long-standing ask: to have the ability of send subsequent notifications to a slack thread instead of channel, which, with 48 👍 is the most upvoted open issue in this repo.

The PR adds a message_strategy field to the Slack notifier with three values: new (default, no change), update (replaces the existing update_message bool), and thread —where subsequent notifications for an alert group are posted as threaded replies instead of flooding the channel.

The thread mode supports two flavours: a lightweight auto-updating parent summary that shows the alert group's transition history, or a simpler mode where the first alert message becomes the thread parent. There are also opt-in extras: a resolve emoji reaction and a custom resolve color.

Happy to get any feedback or review on this!
Cheers,

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Santiago Fernández Núñez

Software Engineer - Reliability Metrics

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