perf(sentry): Re-add tag for over-limit spans (#44684)

### Summary
This will tag transactions when they are at or over the limit, which
likely means other spans were dropped. Dropped spans affects data
quality when graphing transaction data and debugging, so it's good to be
able to separate events along these lines.

Fixes https://github.com/getsentry/sentry/pull/44671 to push it back out

src/sentry/utils/sdk.py

@@ -254,6 +254,12 @@ def traces_sampler(sampling_context):
     return float(settings.SENTRY_BACKEND_APM_SAMPLING or 0)
 
 
+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
+    return event
+
+
 # Patches transport functions to add metrics to improve resolution around events sent to our ingest.
 # Leaving this in to keep a permanent measurement of sdk requests vs ingest.
 def patch_transport_for_instrumentation(transport, transport_name):
@@ -289,6 +295,7 @@ def configure_sdk():
         f"backend@{sdk_options['release']}" if "release" in sdk_options else None
     )
     sdk_options["send_client_reports"] = True
+    sdk_options["before_send_transaction"] = before_send_transaction
 
     if upstream_dsn:
         transport = make_transport(get_options(dsn=upstream_dsn, **sdk_options))