perf: build debug log messages lazily so disabled debug logs cost nothing (#27834)

GLOBAL_LOG_LEVEL defaults to INFO, so every log.debug(...) in the backend is discarded, but the message is built first: 187 call sites interpolate their payload into an f-string before the logging call runs, so the work happens on every request and the result is thrown away. The worst one sits in process_chat_payload and stringifies the whole request body, full conversation history included, once per chat completion.

That one line with DEBUG disabled, CPython 3.12:

| conversation | payload | before   | after   |
| ------------ | ------- | -------- | ------- |
| 4 messages   | 1.2 kB  | 3.4 us   | 0.07 us |
| 20 messages  | 17 kB   | 24.8 us  | 0.07 us |
| 60 messages  | 123 kB  | 216.6 us | 0.07 us |

The lazy form log.debug('form_data: %s', form_data) hands the payload to record.getMessage(), which the InterceptHandler only reaches once a record has passed the level check. With DEBUG enabled the emitted lines are byte-identical, f'{x=}' sites included: those map to %r. MistralLoader._debug_log callers get the same treatment, since that wrapper already forwards *args.

backend/open_webui/utils/chat.py

@@ -115,7 +115,7 @@ async def generate_direct_chat_completion(
                             else:
                                 yield f'data: {data}\n\n'
                 except Exception as e:
-                    log.debug(f'Error in event generator: {e}')
+                    log.debug('Error in event generator: %s', e)
                     pass
 
             # Define a background task to run the event generator
@@ -155,7 +155,7 @@ async def generate_chat_completion(
     bypass_filter: bool = False,
     bypass_system_prompt: bool = False,
 ):
-    log.debug(f'generate_chat_completion: {form_data}')
+    log.debug('generate_chat_completion: %s', form_data)
     if BYPASS_MODEL_ACCESS_CONTROL:
         bypass_filter = True
 
@@ -184,7 +184,7 @@ async def generate_chat_completion(
             **dict(request.app.state.MODELS.items()),
             request.state.model['id']: request.state.model,
         }
-        log.debug(f'direct connection to model: {request.state.model["id"]}')
+        log.debug('direct connection to model: %s', request.state.model['id'])
     else:
         models = request.app.state.MODELS