chore: fix metering format

Author: Daniel Lorch

lib/idp_common_pkg/idp_common/extraction/service.py

@@ -819,10 +819,12 @@ def process_document_section(self, document: Document, section_id: str) -> Docum
                 # Create empty result structure without invoking LLM
                 extracted_fields = {}
                 metering = {
-                    "input_tokens": 0,
-                    "output_tokens": 0,
-                    "invocation_count": 0,
-                    "total_cost": 0.0,
+                    f"Extraction/{self.config.extraction.model}": {
+                        "input_tokens": 0,
+                        "output_tokens": 0,
+                        "invocation_count": 0,
+                        "total_cost": 0.0,
+                    }
                 }
                 total_duration = 0.0
                 parsing_succeeded = True