feat(metric_alerts): Allow trigger thresholds to be float (#19075)

This updates the data model to convert the threshold fields from int -> float.

The data migration should be fine since this table is small/not on a hot path. I tested having code
where the model still has `models.IntegerField` defined, but the database has `double`, and
everything continued to work as usual - fetches and updates all worked fine and didn't cause type
conversion errors.

migrations_lockfile.txt

@@ -10,7 +10,7 @@ auth: 0008_alter_user_username_max_length
 contenttypes: 0002_remove_content_type_name
 jira_ac: 0001_initial
 nodestore: 0001_initial
-sentry: 0081_add_integraiton_upgrade_audit_log
+sentry: 0082_alert_rules_threshold_float
 sessions: 0001_initial
 sites: 0002_alter_domain_unique
 social_auth: 0001_initial

src/sentry/incidents/models.py

@@ -469,8 +469,8 @@ class AlertRuleTrigger(Model):
     alert_rule = FlexibleForeignKey("sentry.AlertRule")
     label = models.TextField()
     threshold_type = models.SmallIntegerField()
-    alert_threshold = models.IntegerField()
-    resolve_threshold = models.IntegerField(null=True)
+    alert_threshold = models.FloatField()
+    resolve_threshold = models.FloatField(null=True)
     triggered_incidents = models.ManyToManyField(
         "sentry.Incident", related_name="triggers", through=IncidentTrigger
     )

src/sentry/migrations/0082_alert_rules_threshold_float.py

@@ -0,0 +1,42 @@
+# -*- coding: utf-8 -*-
+# Generated by Django 1.11.29 on 2020-05-29 20:19
+from __future__ import unicode_literals
+
+from django.db import migrations, models
+
+
+class Migration(migrations.Migration):
+    # This flag is used to mark that a migration shouldn't be automatically run in
+    # production. We set this to True for operations that we think are risky and want
+    # someone from ops to run manually and monitor.
+    # General advice is that if in doubt, mark your migration as `is_dangerous`.
+    # Some things you should always mark as dangerous:
+    # - Large data migrations. Typically we want these to be run manually by ops so that
+    #   they can be monitored. Since data migrations will now hold a transaction open
+    #   this is even more important.
+    # - Adding columns to highly active tables, even ones that are NULL.
+    is_dangerous = False
+
+    # This flag is used to decide whether to run this migration in a transaction or not.
+    # By default we prefer to run in a transaction, but for migrations where you want
+    # to `CREATE INDEX CONCURRENTLY` this needs to be set to False. Typically you'll
+    # want to create an index concurrently when adding one to an existing table.
+    atomic = True
+
+
+    dependencies = [
+        ('sentry', '0081_add_integraiton_upgrade_audit_log'),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name='alertruletrigger',
+            name='alert_threshold',
+            field=models.FloatField(),
+        ),
+        migrations.AlterField(
+            model_name='alertruletrigger',
+            name='resolve_threshold',
+            field=models.FloatField(null=True),
+        ),
+    ]

tests/sentry/api/serializers/test_alert_rule_trigger.py

@@ -31,6 +31,14 @@ def test_simple(self):
         result = serialize(trigger)
         self.assert_alert_rule_trigger_serialized(trigger, result)
 
+    def test_decimal(self):
+        alert_rule = self.create_alert_rule()
+        trigger = create_alert_rule_trigger(
+            alert_rule, "hi", AlertRuleThresholdType.ABOVE, 1000.50, 200.70
+        )
+        result = serialize(trigger)
+        self.assert_alert_rule_trigger_serialized(trigger, result)
+
 
 class DetailedAlertRuleTriggerSerializerTest(BaseAlertRuleTriggerSerializerTest, TestCase):
     def test_simple(self):

tests/sentry/incidents/endpoints/test_serializers.py

@@ -29,8 +29,6 @@ def valid_params(self):
             "dataset": QueryDatasets.EVENTS.value,
             "query": "level:error",
             "threshold_type": 0,
-            "resolve_threshold": 1,
-            "alert_threshold": 0,
             "aggregate": "count()",
             "threshold_period": 1,
             "projects": [self.project.slug],
@@ -160,14 +158,27 @@ def test_transaction_dataset(self):
         assert alert_rule.snuba_query.dataset == QueryDatasets.TRANSACTIONS.value
         assert alert_rule.snuba_query.aggregate == "count()"
 
+    def test_decimal(self):
+        params = self.valid_transaction_params.copy()
+        alert_threshold = 0.8
+        resolve_threshold = 0.7
+        params["triggers"][0]["alertThreshold"] = alert_threshold
+        params["triggers"][0]["resolveThreshold"] = resolve_threshold
+        # Drop off the warning trigger
+        params["triggers"].pop()
+        serializer = AlertRuleSerializer(context=self.context, data=self.valid_transaction_params)
+        assert serializer.is_valid(), serializer.errors
+        alert_rule = serializer.save()
+        trigger = alert_rule.alertruletrigger_set.filter(label="critical").get()
+        assert trigger.alert_threshold == alert_threshold
+        assert trigger.resolve_threshold == resolve_threshold
+
     def test_simple_below_threshold(self):
         payload = {
             "name": "hello_im_a_test",
             "time_window": 10,
             "query": "level:error",
             "threshold_type": 0,
-            "resolve_threshold": 1,
-            "alert_threshold": 0,
             "aggregate": "count()",
             "threshold_period": 1,
             "projects": [self.project.slug],