fix(uptime): fix cron monitor incident timing inconsistency

Ensure we use the same kind of 'time' for timestamps when computing the start and end of crons open periods.
We currently use the consumer processing time for the start of an open period, but use the relay time for the close of it.   In the event that a cron start is massively delayed due to backlogging, we wind up with the end coming before the start.  This corrects that inconsistency by using the relay time for the open period start.

Author: klochek

src/sentry/monitors/logic/incident_occurrence.py

@@ -4,7 +4,7 @@
 import uuid
 from collections import Counter
 from collections.abc import Mapping, Sequence
-from datetime import datetime, timezone
+from datetime import datetime
 from typing import TYPE_CHECKING
 
 from arroyo import Topic as ArroyoTopic
@@ -138,8 +138,6 @@ def send_incident_occurrence(
     """
     monitor_env = failed_checkin.monitor_environment
 
-    current_timestamp = datetime.now(timezone.utc)
-
     # Get last successful check-in to show in evidence display
     last_successful_checkin_timestamp = "Never"
     last_successful_checkin = monitor_env.get_last_successful_checkin()
@@ -217,9 +215,7 @@ def send_incident_occurrence(
             "monitor.slug": str(monitor_env.monitor.slug),
             "monitor.incident": str(incident.id),
         },
-        # option 1: timestamp <- received.isoformat().  This seems reasonable, because the cron incident starts not when
-        # we've processed it, but when relay got the message (which is approximately when the cron failure occurred.)
-        "timestamp": current_timestamp.isoformat(),
+        "timestamp": received.isoformat(),
     }
 
     if trace_id:
@@ -295,8 +291,6 @@ def resolve_incident_group(incident: MonitorIncident, project_id: int) -> None:
         project_id=project_id,
         new_status=GroupStatus.RESOLVED,
         new_substatus=None,
-        # option 2: make this current_time.  Then, because of monitor ordering guarantees, this will always be ahead of
-        # the start time
         update_date=incident.resolving_timestamp,
     )
     produce_occurrence_to_kafka(