perf: stop formatting every exported log record twice under OTEL log export (#27840)

With ENABLE_OTEL and ENABLE_OTEL_LOGS set, InterceptHandler builds the message once for loguru and then hands the same LogRecord to the OpenTelemetry handler, whose _translate calls record.getMessage() a second time. That used to be free, because the message was already a finished f-string with nothing to substitute. Now that log calls pass lazy %-args, the second call re-runs the whole interpolation, so every exported record is formatted twice.

The two getMessage() calls on a 78 kB retrieval record:

    before  373.0 us
    after     0.1 us

Stamping the built message back onto the record makes the second call a plain string return. msg and args are both in OpenTelemetry's _RESERVED_ATTRS, so neither ever reaches the exported attributes. The isinstance guard matters: _translate exports a non-str msg such as the dicts routers/audio.py logs as a typed body rather than a string, so those records are left untouched, and they have no %-args to format twice anyway. Body, attributes and severity were compared against LoggingHandler._translate for str, dict, list, int, None, exception and exc_info records.

backend/open_webui/utils/logger.py

@@ -112,10 +112,14 @@ class InterceptHandler(logging.Handler):
             frame = frame.f_back
             depth += 1
 
-        logger.opt(depth=depth, exception=record.exc_info).bind(**self._get_extras()).log(level, record.getMessage())
+        message = record.getMessage()
+        logger.opt(depth=depth, exception=record.exc_info).bind(**self._get_extras()).log(level, message)
         if ENABLE_OTEL and ENABLE_OTEL_LOGS:
             from open_webui.utils.telemetry.logs import otel_handler
 
+            # reuse the message we built so %-args format once; a non-str msg is left alone, otel exports it structured
+            if isinstance(record.msg, str):
+                record.msg, record.args = message, None
             otel_handler.emit(record)
 
     def _get_extras(self):