chore(unmerge): Add more context when logging (#101190)

This would have helped us when investigating some unmerge customer
issue.

Author: armenzg

src/sentry/tasks/unmerge.py

@@ -168,10 +168,11 @@ def migrate_events(
     opt_destination_id: int | None,
     opt_eventstream_state: Mapping[str, Any] | None,
 ) -> tuple[int, Mapping[str, Any]]:
+    extra = {"source_id": args.source_id, "project_id": project.id}
     logger.info(
         "migrate_events.start",
         extra={
-            "source_id": args.source_id,
+            **extra,
             "opt_destination_id": opt_destination_id,
             "migrate_args": args,
         },
@@ -202,7 +203,7 @@ def migrate_events(
         destination.update(**get_group_backfill_attributes(caches, destination, events))
 
     update_open_periods(source, destination)
-    logger.info("migrate_events.migrate", extra={"destination_id": destination_id})
+    logger.info("migrate_events.migrate", extra={**extra, "destination_id": destination_id})
 
     if isinstance(args, InitialUnmergeArgs) or opt_eventstream_state is None:
         eventstream_state = args.replacement.start_snuba_replacement(
@@ -522,6 +523,8 @@ def unlock_hashes(project_id: int, locked_primary_hashes: Sequence[str]) -> None
 )
 def unmerge(*posargs: Any, **kwargs: Any) -> None:
     args = UnmergeArgsBase.parse_arguments(*posargs, **kwargs)
+    extra = {"source_id": args.source_id, "project_id": args.project_id}
+    logger.info("unmerge.start.task", extra=extra)
 
     source = Group.objects.get(project_id=args.project_id, id=args.source_id)
 
@@ -552,22 +555,14 @@ def unmerge(*posargs: Any, **kwargs: Any) -> None:
     # Convert Event objects to GroupEvent objects
     events: list[GroupEvent] = [event.for_group(source) for event in raw_events]
     # Log info related to this unmerge
-    logger.info(
-        "unmerge.check",
-        extra={
-            "source_id": source.id,
-            "num_events": len(events),
-        },
-    )
+    logger.info("unmerge.check", extra={**extra, "num_events": len(events)})
 
     # If there are no more events to process, we're done with the migration.
     if not events:
         unlock_hashes(args.project_id, locked_primary_hashes)
-        for unmerge_key, (group_id, eventstream_state) in args.destinations.items():
+        for unmerge_key, (_, eventstream_state) in args.destinations.items():
             logger.warning(
-                "Unmerge complete (eventstream state: %s)",
-                eventstream_state,
-                extra={"source_id": source.id},
+                "Unmerge complete (eventstream state: %s)", eventstream_state, extra=extra
             )
             if eventstream_state:
                 args.replacement.stop_snuba_replacement(eventstream_state)
@@ -597,7 +592,7 @@ def unmerge(*posargs: Any, **kwargs: Any) -> None:
     logger.info(
         "unmerge.destinations",
         extra={
-            "source_id": source.id,
+            **extra,
             "source_events": len(source_events),
             "destination_events": len(destination_events),
             "source_fields_reset": source_fields_reset,