Add limit to API for Alert Rules (#70596)

## Description
Fixes: #67195

Adds the `x-sentry-rule-limit` header to the list views for alert rules
(both the combined-rules and metric alert api).

I tried adding an indicator to the UI, but it was pretty difficult to
tell which page we're on in the UI, since we're handling it on the
backend with a `Link`. Rather than blocking the API fix on trying to
figure out the UI, i figure it's better to get this out and we can add
to the UI later.

![Screenshot 2024-05-09 at 11 39
43 AM](https://github.com/getsentry/sentry/assets/1569818/3cb0d796-95ad-4f87-8344-88d9599b6c79)

src/sentry/api/helpers/constants.py

@@ -0,0 +1,2 @@
+MAX_QUERY_SUBSCRIPTIONS_HEADER = "X-Sentry-Alert-Rule-Limit"
+ALERT_RULES_COUNT_HEADER = "X-Sentry-Alert-Rule-Hits"

src/sentry/incidents/endpoints/organization_alert_rule_index.py

@@ -1,6 +1,7 @@
 from copy import deepcopy
 from datetime import UTC, datetime
 
+from django.conf import settings
 from django.db.models import DateTimeField, IntegerField, OuterRef, Q, Subquery, Value
 from django.db.models.functions import Coalesce
 from drf_spectacular.utils import extend_schema, extend_schema_serializer
@@ -15,6 +16,7 @@
 from sentry.api.bases.organization import OrganizationAlertRulePermission, OrganizationEndpoint
 from sentry.api.exceptions import ResourceDoesNotExist
 from sentry.api.fields.actor import ActorField
+from sentry.api.helpers.constants import ALERT_RULES_COUNT_HEADER, MAX_QUERY_SUBSCRIPTIONS_HEADER
 from sentry.api.paginator import (
     CombinedQuerysetIntermediary,
     CombinedQuerysetPaginator,
@@ -70,7 +72,7 @@ def fetch_metric_alert(self, request, organization, project=None):
         if monitor_type is not None:
             alert_rules = alert_rules.filter(monitor_type=monitor_type)
 
-        return self.paginate(
+        response = self.paginate(
             request,
             queryset=alert_rules,
             order_by="-date_added",
@@ -79,6 +81,10 @@ def fetch_metric_alert(self, request, organization, project=None):
             default_per_page=25,
         )
 
+        response[ALERT_RULES_COUNT_HEADER] = len(alert_rules)
+        response[MAX_QUERY_SUBSCRIPTIONS_HEADER] = settings.MAX_QUERY_SUBSCRIPTIONS_PER_ORG
+        return response
+
     def create_metric_alert(self, request, organization, project=None):
         if not features.has("organizations:incidents", organization, actor=request.user):
             raise ResourceDoesNotExist
@@ -278,7 +284,8 @@ def get(self, request: Request, organization) -> Response:
             case_insensitive=case_insensitive,
         )
         response["X-Sentry-Issue-Rule-Hits"] = issue_rules_count
-        response["X-Sentry-Alert-Rule-Hits"] = alert_rules_count
+        response[ALERT_RULES_COUNT_HEADER] = alert_rules_count
+        response[MAX_QUERY_SUBSCRIPTIONS_HEADER] = settings.MAX_QUERY_SUBSCRIPTIONS_PER_ORG
         return response
 
 

tests/sentry/incidents/endpoints/test_organization_alert_rule_index.py

@@ -9,6 +9,7 @@
 from rest_framework import status
 
 from sentry import audit_log
+from sentry.api.helpers.constants import ALERT_RULES_COUNT_HEADER, MAX_QUERY_SUBSCRIPTIONS_HEADER
 from sentry.api.serializers import serialize
 from sentry.incidents.models.alert_rule import (
     AlertRule,
@@ -122,6 +123,18 @@ def test_filter_by_monitor_type(self):
         assert serialize([alert_rule2]) not in resp.data
         assert resp.data == serialize([alert_rule1])
 
+    def test_response_headers(self):
+        self.create_team(organization=self.organization, members=[self.user])
+        self.create_alert_rule(monitor_type=AlertRuleMonitorType.ACTIVATED)
+        self.create_alert_rule(monitor_type=AlertRuleMonitorType.CONTINUOUS)
+        self.login_as(self.user)
+
+        with self.feature("organizations:incidents"):
+            resp = self.get_response(self.organization.slug)
+
+        assert resp[ALERT_RULES_COUNT_HEADER] == "2"
+        assert resp[MAX_QUERY_SUBSCRIPTIONS_HEADER] == "1000"
+
     def test_simple_with_activation(self):
         self.create_team(organization=self.organization, members=[self.user])
         alert_rule = self.create_alert_rule()