chore(enterprise): Track number of spans per Python transaction (#44760)

Author: dashed

src/sentry/utils/sdk.py

@@ -256,7 +256,11 @@ def traces_sampler(sampling_context):
 
 def before_send_transaction(event, _):
     # Occasionally the span limit is hit and we drop spans from transactions, this helps find transactions where this occurs.
-    event["tags"]["spans_over_limit"] = len(event["spans"]) >= 1000
+    num_of_spans = len(event["spans"])
+    event["tags"]["spans_over_limit"] = num_of_spans >= 1000
+    if not event["measurements"]:
+        event["measurements"] = {}
+    event["measurements"]["num_of_spans"] = {"value": num_of_spans}
     return event