Implement get_non_flushed_event_count() for MemoryCache

Closes #88.

Author: eht16

logstash_async/cache.py

@@ -61,3 +61,12 @@ def expire_events(self):
         :return:
         """
         pass
+
+    # ----------------------------------------------------------------------
+    @abstractmethod
+    def get_non_flushed_event_count(self):
+        """Determine the count of pending events in the cache which need to be sent to Logstash.
+
+        :return: Count of pending events
+        """
+        pass

logstash_async/memory_cache.py

@@ -93,3 +93,7 @@ def _delete_events(self, ids_to_delete):
                 self.logger.warning(
                     "Could not delete event with id %s. It does not appear to be in the cache.",
                     event_id)
+
+    # ----------------------------------------------------------------------
+    def get_non_flushed_event_count(self):
+        return len([event for event in self._cache.values() if not event['pending_delete']])