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.
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