Limit on Attach To action in Slack (#2049)

# What this PR does
Reduce number of alert groups returned for grouping on slack request to
20 to avoid event trigger expiry.

## Which issue(s) this PR fixes
https://github.com/grafana/oncall-private/issues/1835

## Checklist

- [ ] Unit, integration, and e2e (if applicable) tests updated
- [x] Documentation added (or `pr:no public docs` PR label added if not
required)
- [x] `CHANGELOG.md` updated (or `pr:no changelog` PR label added if not
required)

---------

Co-authored-by: Joey Orlando <joey.orlando@grafana.com>

CHANGELOG.md

@@ -37,6 +37,7 @@ and this project adheres to [Semantic Versioning](https://semver.org/spec/v2.0.0
 - Properly address `Organization.DoesNotExist` exceptions thrown which result in HTTP 500 for the Slack `interactive_api_endpoint`
   endpoint by @joeyorlando ([#2040](https://github.com/grafana/oncall/pull/2040))
 - Fix issue when trying to sync Grafana contact point and config receivers miss a key ([#2046](https://github.com/grafana/oncall/pull/2046))
+- Reduce number of alert groups returned by `Attach To` in slack to avoid event trigger timeout @mderynck ([#2049](https://github.com/grafana/oncall/pull/2049))
 
 ### Changed
 

engine/apps/slack/scenarios/distribute_alerts.py

@@ -37,6 +37,8 @@ from common.utils import clean_markup, is_string_with_visible_characters
 
 from .step_mixins import CheckAlertIsUnarchivedMixin, IncidentActionsAccessControlMixin
 
+ATTACH_TO_ALERT_GROUPS_LIMIT = 20
+
 logger = logging.getLogger(__name__)
 logger.setLevel(logging.DEBUG)
 
@@ -398,7 +400,7 @@ class SelectAttachGroupStep(
             .order_by("-pk")
         )
 
-        for alert_group_to_attach in alert_groups_queryset[:60]:
+        for alert_group_to_attach in alert_groups_queryset[:ATTACH_TO_ALERT_GROUPS_LIMIT]:
             # long_verbose_name_without_formatting was removed from here because it increases queries count due to
             # alerts.first().
             # alert_group_to_attach.alerts.exists() and alerts.all()[0] don't make additional queries to db due to
@@ -435,7 +437,7 @@ class SelectAttachGroupStep(
                             "text": "Attach to...",
                         },
                         "action_id": AttachGroupStep.routing_uid(),
-                        "options": collected_options[:60],
+                        "options": collected_options[:ATTACH_TO_ALERT_GROUPS_LIMIT],
                     },
                     "label": {
                         "type": "plain_text",