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/routers/tasks.py

@@ -144,7 +144,7 @@ async def generate_title(request: Request, form_data: dict, user=Depends(get_ver
         models,
     )
 
-    log.debug(f'generating chat title using model {task_model_id} for user {user.email} ')
+    log.debug('generating chat title using model %s for user %s ', task_model_id, user.email)
 
     title_template = await Config.get('task.title.prompt_template')
     if title_template != '':
@@ -223,7 +223,7 @@ async def generate_follow_ups(request: Request, form_data: dict, user=Depends(ge
         models,
     )
 
-    log.debug(f'generating chat title using model {task_model_id} for user {user.email} ')
+    log.debug('generating chat title using model %s for user %s ', task_model_id, user.email)
 
     follow_up_template = await Config.get('task.follow_up.prompt_template')
     if follow_up_template != '':
@@ -293,7 +293,7 @@ async def generate_chat_tags(request: Request, form_data: dict, user=Depends(get
         models,
     )
 
-    log.debug(f'generating chat tags using model {task_model_id} for user {user.email} ')
+    log.debug('generating chat tags using model %s for user %s ', task_model_id, user.email)
 
     tags_template = await Config.get('task.tags.prompt_template')
     if tags_template != '':
@@ -357,7 +357,7 @@ async def generate_image_prompt(request: Request, form_data: dict, user=Depends(
         models,
     )
 
-    log.debug(f'generating image prompt using model {task_model_id} for user {user.email} ')
+    log.debug('generating image prompt using model %s for user %s ', task_model_id, user.email)
 
     image_prompt_template = await Config.get('task.image.prompt_template')
     if image_prompt_template != '':
@@ -439,7 +439,7 @@ async def generate_queries(request: Request, form_data: dict, user=Depends(get_v
         models,
     )
 
-    log.debug(f'generating {type} queries using model {task_model_id} for user {user.email}')
+    log.debug('generating %s queries using model %s for user %s', type, task_model_id, user.email)
 
     query_template = await Config.get('task.query.prompt_template')
     if query_template.strip() != '':
@@ -520,7 +520,7 @@ async def generate_autocompletion(request: Request, form_data: dict, user=Depend
         models,
     )
 
-    log.debug(f'generating autocompletion using model {task_model_id} for user {user.email}')
+    log.debug('generating autocompletion using model %s for user %s', task_model_id, user.email)
 
     autocomplete_template = await Config.get('task.autocomplete.prompt_template')
     if autocomplete_template.strip() != '':
@@ -584,7 +584,7 @@ async def generate_emoji(request: Request, form_data: dict, user=Depends(get_ver
         models,
     )
 
-    log.debug(f'generating emoji using model {task_model_id} for user {user.email} ')
+    log.debug('generating emoji using model %s for user %s ', task_model_id, user.email)
 
     template = DEFAULT_EMOJI_GENERATION_PROMPT_TEMPLATE