fix(tracing): Correct discarded transaction debug message

Transactions that are discarded because `_span_recorder` is `None` can also be discarded because they were not started with `sentry_sdk.start_transaction`. This change updates the debug message accordingly.

Fixes GH-3000

sentry_sdk/tracing.py

@@ -748,7 +748,13 @@ def finish(self, hub=None, end_timestamp=None):
             return None
 
         if self._span_recorder is None:
-            logger.debug("Discarding transaction because sampled = False")
+            # Explicit check against False needed because self.sampled might be None
+            if self.sampled is False:
+                logger.debug("Discarding transaction because sampled = False")
+            else:
+                logger.debug(
+                    "Discarding transaction because it was not started with sentry_sdk.start_transaction"
+                )
 
             # This is not entirely accurate because discards here are not
             # exclusively based on sample rate but also traces sampler, but