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/retrieval/vector/dbs/valkey.py

@@ -390,7 +390,7 @@ class ValkeyClient(VectorDBBase):
             )
         except g['RequestError'] as e:
             if 'already exists' in str(e).lower():
-                log.debug(f'Index {index_name} already exists, skipping creation.')
+                log.debug('Index %s already exists, skipping creation.', index_name)
             else:
                 raise
 
@@ -458,7 +458,7 @@ class ValkeyClient(VectorDBBase):
             self._g['glide_ft'].dropindex(self.client, index_name)
             log.info(f'Dropped index {index_name}')
         except self._g['RequestError'] as e:
-            log.debug(f'Could not drop index {index_name}: {e}')
+            log.debug('Could not drop index %s: %s', index_name, e)
 
         self._delete_keys_by_prefix(self._key_prefix(collection_name))
 
@@ -492,7 +492,7 @@ class ValkeyClient(VectorDBBase):
             }
             self.batch_client.hset(self._item_key(collection_name, item['id']), mapping)
 
-        log.debug(f'Inserted {len(items)} items into collection {collection_name}')
+        log.debug('Inserted %s items into collection %s', len(items), collection_name)
 
     def upsert(self, collection_name: str, items: list[VectorItem]):
         self.insert(collection_name, items)