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/webhook.py

@@ -29,7 +29,7 @@ def _event_text(message: str, description: str | None = None, event_data: dict |
 
 async def post_webhook(name: str, url: str, message: str, event_data: dict, description: str | None = None) -> bool:
     try:
-        log.debug(f'post_webhook: {url}, {message}, {event_data}')
+        log.debug('post_webhook: %s, %s, %s', url, message, event_data)
         # Block private-IP / loopback / cloud-metadata targets — the URL is
         # caller-controlled (user notification settings under
         # ENABLE_USER_WEBHOOKS, automation notification triggers).
@@ -83,7 +83,7 @@ async def post_webhook(name: str, url: str, message: str, event_data: dict, desc
         else:
             payload = event_data
 
-        log.debug(f'payload: {payload}')
+        log.debug('payload: %s', payload)
         async with get_ssrf_safe_session() as session:
             async with session.post(
                 url,
@@ -93,7 +93,7 @@ async def post_webhook(name: str, url: str, message: str, event_data: dict, desc
             ) as r:
                 r_text = await r.text()
                 r.raise_for_status()
-                log.debug(f'r.text: {r_text}')
+                log.debug('r.text: %s', r_text)
 
         return True
     except Exception as e: