fix: Send client reports for span recorder overflow (#5310)

### Description
We weren't emitting client reports for spans dropped because they were
over the span recorder limit.

Disclaimer: vibe coded but refactored and live tested by hand

Tested 3 times with an app with `max_spans=3` and 10 spans in a
transaction
<img width="1336" height="410" alt="Screenshot 2026-01-14 at 14 41 08"
src="https://github.com/user-attachments/assets/f1c0add8-7d56-427b-aa3c-0e25e917038f"
/>


#### Issues
Closes #5308

#### Reminders
- Please add tests to validate your changes, and lint your code using
`tox -e linters`.
- Add GH Issue ID _&_ Linear ID (if applicable)
- PR title should use [conventional
commit](https://develop.sentry.dev/engineering-practices/commit-messages/#type)
style (`feat:`, `fix:`, `ref:`, `meta:`)
- For external contributors:
[CONTRIBUTING.md](https://github.com/getsentry/sentry-python/blob/master/CONTRIBUTING.md),
[Sentry SDK development docs](https://develop.sentry.dev/sdk/), [Discord
community](https://discord.gg/Ww9hbqr)

Author: sentrivana

sentry_sdk/client.py

@@ -527,12 +527,21 @@ def _prepare_event(
             spans_delta = spans_before - len(
                 cast(List[Dict[str, object]], event.get("spans", []))
             )
-            if is_transaction and spans_delta > 0 and self.transport is not None:
-                self.transport.record_lost_event(
-                    "event_processor", data_category="span", quantity=spans_delta
-                )
+            span_recorder_dropped_spans: int = event.pop("_dropped_spans", 0)
+
+            if is_transaction and self.transport is not None:
+                if spans_delta > 0:
+                    self.transport.record_lost_event(
+                        "event_processor", data_category="span", quantity=spans_delta
+                    )
+                if span_recorder_dropped_spans > 0:
+                    self.transport.record_lost_event(
+                        "buffer_overflow",
+                        data_category="span",
+                        quantity=span_recorder_dropped_spans,
+                    )
 
-            dropped_spans: int = event.pop("_dropped_spans", 0) + spans_delta
+            dropped_spans: int = span_recorder_dropped_spans + spans_delta
             if dropped_spans > 0:
                 previous_total_spans = spans_before + dropped_spans
             if scope._n_breadcrumbs_truncated > 0:

tests/tracing/test_misc.py

@@ -37,6 +37,26 @@ def test_span_trimming(sentry_init, capture_events):
     assert "dropped_spans" not in event
 
 
+def test_span_trimming_produces_client_report(
+    sentry_init, capture_events, capture_record_lost_event_calls
+):
+    sentry_init(traces_sample_rate=1.0, _experiments={"max_spans": 3})
+    events = capture_events()
+    record_lost_event_calls = capture_record_lost_event_calls()
+
+    with start_transaction(name="hi"):
+        for i in range(10):
+            with start_span(op="foo{}".format(i)):
+                pass
+
+    (event,) = events
+
+    assert len(event["spans"]) == 3
+
+    # 7 spans were dropped (10 total - 3 kept = 7 dropped)
+    assert ("buffer_overflow", "span", None, 7) in record_lost_event_calls
+
+
 def test_span_data_scrubbing_and_trimming(sentry_init, capture_events):
     sentry_init(traces_sample_rate=1.0, _experiments={"max_spans": 3})
     events = capture_events()