ref(grouping): Clarify outcome tagging on group-creation DB transaction (#68241)

During ingest, once we've calculated the event's hashes and checked them against existing ones and not discovered any matches, we enter a branch in the logic where we either create a new group or fail to acquire the lock and wait while another event with the same hash going through ingest ever-so-slightly ahead creates the group, which we then use. We time this branch with both a span and a timer metric, and tag them both so that we know whether we created a group or had to wait for the lock (and then used the relevant newly-created group).

All good, except that we tag the latter case as a `"no_group"` case, which - while technically accurate, in that we create no group - isn't super clear. This switches to recording that result as `"wait-for-lock"`, so it's more obvious what's happening. While I was there, I also changed the tag's name from `"create_group_transaction.outcome"` to just `"outcome"` - since it only ever appears on the `event_manager.create_group_transaction` span/metric, there's no need to namespace it further.

src/sentry/event_manager.py

@@ -1457,8 +1457,8 @@ def _save_aggregate(
             metrics.timer("event_manager.create_group_transaction") as metric_tags,
             transaction.atomic(router.db_for_write(GroupHash)),
         ):
-            span.set_tag("create_group_transaction.outcome", "no_group")
-            metric_tags["create_group_transaction.outcome"] = "no_group"
+            span.set_tag("outcome", "wait_for_lock")
+            metric_tags["outcome"] = "wait_for_lock"
 
             all_grouphash_ids = [h.id for h in flat_grouphashes]
             if root_hierarchical_grouphash is not None:
@@ -1496,8 +1496,8 @@ def _save_aggregate(
                 is_new = True
                 is_regression = False
 
-                span.set_tag("create_group_transaction.outcome", "new_group")
-                metric_tags["create_group_transaction.outcome"] = "new_group"
+                span.set_tag("outcome", "new_group")
+                metric_tags["outcome"] = "new_group"
                 record_calculation_metric_with_result(
                     project=project,
                     has_secondary_hashes=has_secondary_hashes,
@@ -1782,8 +1782,8 @@ def create_group_with_grouphashes(
         metrics.timer("event_manager.create_group_transaction") as metrics_timer_tags,
         transaction.atomic(router.db_for_write(GroupHash)),
     ):
-        span.set_tag("create_group_transaction.outcome", "no_group")
-        metrics_timer_tags["create_group_transaction.outcome"] = "no_group"
+        span.set_tag("outcome", "wait_for_lock")
+        metrics_timer_tags["outcome"] = "wait_for_lock"
 
         # If we're in this branch, we checked our grouphashes and didn't find one with a group
         # attached. We thus want to create a new group, but we need to guard against another
@@ -1810,8 +1810,8 @@ def create_group_with_grouphashes(
         # If we still haven't found a matching grouphash, we're now safe to go ahead and create
         # the group.
         if existing_grouphash is None:
-            span.set_tag("create_group_transaction.outcome", "new_group")
-            metrics_timer_tags["create_group_transaction.outcome"] = "new_group"
+            span.set_tag("outcome", "new_group")
+            metrics_timer_tags["outcome"] = "new_group"
             record_new_group_metrics(event)
 
             group = _create_group(project, event, **group_processing_kwargs)