feat: allow Slack receiver to edit existing messages

[chazapp]

Details

This PR adds an update_message config to Slack notifiers, that enables AlertManager to edit previously sent notifications rather than sending new ones for alert status change. It uses the nflog Store introduced in v0.31.0 to persist Slack's message Timestamp and channel ID. If retrieved, the feature will change the API endpoint to chat.update and the request payload to attempt to edit the message.

The PR is backwards compatible and this new feature requires opt-in.

---

Testing

Test Config

global:
  resolve_timeout: 5m

route:
  receiver: 'slack'
  group_by: ['alertname']
  group_wait: 10s
  group_interval: 10s
  repeat_interval: 12h

receivers:
  - name: 'slack'
    slack_configs:
      - send_resolved: true
        api_url: 'https://slack.com/api/chat.postMessage'
        http_config:
          authorization:
            credentials: 'xoxb-....'
        channel: '#test-channel'
        update_message: true  # This enables message updates! 
        title: '{{ .GroupLabels.alertname }} - {{ .Status | toUpper }}'
        text: |
          {{ if eq .Status "firing" }}
          🔥 *{{ .Alerts.Firing | len }} alert(s) firing*
          {{ else }}
          ✅ *All alerts resolved*
          {{ end }}
          
          {{ range .Alerts }}
          • {{ if .Annotations.summary }}{{ .Annotations.summary }}{{ else }}{{ .Labels.alertname }}{{ end }}
          {{ end }}
        
        color: '{{ if eq .Status "firing" }}danger{{ else }}good{{ end }}'

Test script

#!/bin/bash

## Sends an Alertmanager alert
## Usage:
## ./alert.sh
## ./alert.sh resolve

case $1 in
    "")
	curl -X POST http://localhost:9093/api/v2/alerts -H 'Content-Type: application/json' -d '[{
	  "labels": {"alertname": "TestAlert", "severity": "warning"},
	    "annotations": {"summary": "This alert will resolve in the same message"}
	}]';;
    "resolve")
	curl -X POST http://localhost:9093/api/v2/alerts -H 'Content-Type: application/json' -d '[{
	  "labels": {"alertname": "TestAlert", "severity": "warning"},
	    "annotations": {"summary": "This alert will resolve in the same message"},
	     "endsAt": "'$(date -u +%Y-%m-%dT%H:%M:%SZ)'"
	}]';
esac

---

Misc

Special thanks to @kruchkov-alexandr for preliminary PR (#4682) and @Spaceman1701 for the nflog data store implementation (#4899).

[siavashs]

Thanks, this LGTM in general.
Just added few comments to potentially improve the logic and config.

[siavashs]

Thanks for addressing the previous comments, I added a few more.

[siavashs]

LGTM, Thanks!

[Spaceman1701]

This looks very good to me!

I don't have any problems with the implementation or logic, just a few small code-style requests.

I don't think it should be (or needs to be) included in this PR, but it might be nice for a future change to provide some way for the user's template to discover it's updating an existing message. Unfortunately I think this will be a little complicated, so we can save it for later.