Fix: Add method to merge conversation stats (#10667)

Co-authored-by: openhands <openhands@all-hands.dev>

openhands/server/services/conversation_stats.py

@@ -101,3 +101,60 @@ class ConversationStats:
             del self.restored_metrics[service_id]
 
         self.service_to_metrics[service_id] = llm.metrics
+
+    def merge_and_save(self, conversation_stats: 'ConversationStats'):
+        """
+        Merge restored metrics from another ConversationStats into this one.
+
+        Important:
+        - This method is intended to be used immediately after restoring metrics from
+          storage, before any LLM services are registered. In that state, only
+          `restored_metrics` should contain entries and `service_to_metrics` should
+          be empty. If either side has entries in `service_to_metrics`, we log an
+          error but continue execution.
+
+        Behavior:
+        - Drop entries with zero accumulated_cost from both `restored_metrics` dicts
+          (self and incoming) before merging.
+        - Merge only `restored_metrics`. For duplicate keys, the incoming
+          `conversation_stats.restored_metrics` overwrites existing entries.
+        - Do NOT merge `service_to_metrics` here.
+        - Persist results by calling save_metrics().
+        """
+
+        # If either side has active service metrics, log an error but proceed
+        if self.service_to_metrics or conversation_stats.service_to_metrics:
+            logger.error(
+                'merge_and_save should be used only when service_to_metrics are empty; '
+                'found active service metrics during merge. Proceeding anyway.',
+                extra={
+                    'conversation_id': self.conversation_id,
+                    'self_service_to_metrics_keys': list(
+                        self.service_to_metrics.keys()
+                    ),
+                    'incoming_service_to_metrics_keys': list(
+                        conversation_stats.service_to_metrics.keys()
+                    ),
+                },
+            )
+
+        # Drop zero-cost entries from restored metrics only
+        def _drop_zero_cost(d: dict[str, Metrics]) -> None:
+            to_delete = [
+                k for k, v in d.items() if getattr(v, 'accumulated_cost', 0) == 0
+            ]
+            for k in to_delete:
+                del d[k]
+
+        _drop_zero_cost(self.restored_metrics)
+        _drop_zero_cost(conversation_stats.restored_metrics)
+
+        # Merge restored metrics, allowing incoming to overwrite
+        self.restored_metrics.update(conversation_stats.restored_metrics)
+
+        # Save merged state
+        self.save_metrics()
+        logger.info(
+            'Merged conversation stats',
+            extra={'conversation_id': self.conversation_id},
+        )