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/functions.py

@@ -100,7 +100,7 @@ async def get_function_models(request):
                     log.exception(e)
                     sub_pipes = []
 
-                log.debug(f"get_function_models: function '{pipe.id}' is a manifold of {sub_pipes}")
+                log.debug("get_function_models: function '%s' is a manifold of %s", pipe.id, sub_pipes)
 
                 for p in sub_pipes:
                     sub_pipe_id = f'{pipe.id}.{p["id"]}'
@@ -126,7 +126,10 @@ async def get_function_models(request):
                 pipe_flag = {'type': 'pipe'}
 
                 log.debug(
-                    f"get_function_models: function '{pipe.id}' is a single pipe {{ 'id': {pipe.id}, 'name': {pipe.name} }}"
+                    "get_function_models: function '%s' is a single pipe { 'id': %s, 'name': %s }",
+                    pipe.id,
+                    pipe.id,
+                    pipe.name,
                 )
 
                 pipe_models.append(