feat(migrations): Add new index to IncidentTrigger model (#58467)

This PR adds a new index to `IncidentTrigger` to make querying for the
metric alert rate limiting feature more efficient #58032.

[Query in
question](https://github.com/getsentry/sentry/pull/58032/files#diff-d262481ba0aaae3473d14f62d3cbb554e099bda1093454b7935890139f3fe5a8R558-R563)

Author: isabellaenriquez

migrations_lockfile.txt

@@ -9,5 +9,5 @@ feedback: 0003_feedback_add_env
 hybridcloud: 0005_add_missing_org_integration_scope
 nodestore: 0002_nodestore_no_dictfield
 replays: 0003_add_size_to_recording_segment
-sentry: 0578_add_query_and_users_to_custom_dynamic_sampling_rules
+sentry: 0579_index_incident_trigger
 social_auth: 0002_default_auto_field

src/sentry/incidents/models.py

@@ -523,6 +523,7 @@ class Meta:
         app_label = "sentry"
         db_table = "sentry_incidenttrigger"
         unique_together = (("incident", "alert_rule_trigger"),)
+        index_together = (("alert_rule_trigger", "incident_id"),)
 
 
 class AlertRuleTriggerManager(BaseManager):

src/sentry/migrations/0579_index_incident_trigger.py

@@ -0,0 +1,30 @@
+# Generated by Django 3.2.20 on 2023-10-20 15:31
+
+from django.db import migrations
+
+from sentry.new_migrations.migrations import CheckedMigration
+
+
+class Migration(CheckedMigration):
+    # This flag is used to mark that a migration shouldn't be automatically run in production. For
+    # the most part, this should only be used for operations where it's safe to run the migration
+    # after your code has deployed. So this should not be used for most operations that alter the
+    # schema of a table.
+    # Here are some things that make sense to mark as dangerous:
+    # - Large data migrations. Typically we want these to be run manually by ops so that they can
+    #   be monitored and not block the deploy for a long period of time while they run.
+    # - Adding indexes to large tables. Since this can take a long time, we'd generally prefer to
+    #   have ops run this and not block the deploy. Note that while adding an index is a schema
+    #   change, it's completely safe to run the operation after the code has deployed.
+    is_dangerous = True
+
+    dependencies = [
+        ("sentry", "0578_add_query_and_users_to_custom_dynamic_sampling_rules"),
+    ]
+
+    operations = [
+        migrations.AlterIndexTogether(
+            name="incidenttrigger",
+            index_together={("alert_rule_trigger", "incident_id")},
+        ),
+    ]