Add log message when DedupeIntegration is dropping an error. (#4788)

Make it clearer for the user what is happening.

resolves: PY-1840

Author: antonpirker

sentry_sdk/integrations/dedupe.py

@@ -1,5 +1,5 @@
 import sentry_sdk
-from sentry_sdk.utils import ContextVar
+from sentry_sdk.utils import ContextVar, logger
 from sentry_sdk.integrations import Integration
 from sentry_sdk.scope import add_global_event_processor
 
@@ -37,7 +37,9 @@ def processor(event, hint):
 
             exc = exc_info[1]
             if integration._last_seen.get(None) is exc:
+                logger.info("DedupeIntegration dropped duplicated error event %s", exc)
                 return None
+
             integration._last_seen.set(exc)
             return event