Formatting message does not work

While trying to truncate the message part using formatter. `fluent.handler.FluentRecordFormatter` works normally with all fields except `message`

configuration
```
LOGGING = {
'version': 1,
'disable_existing_loggers': True,
'formatters': {
'verbose': {
'format': "[%(levelname)s %(asctime)s %(module)s -> %(lineno)d] %(message)s",
'datefmt': "%Y-%b-%d %H:%M:%S"
},
'console': {
'format': '[%(levelname)s %(asctime)s] %(message)s',
'datefmt': "%Y-%b-%d %H:%M:%S"
},
'fluent_fmt': {
'()': "fluent.handler.FluentRecordFormatter",
'format': {
'lvl': '%(levelname)s',
'host': '%(hostname)s',
'mod': '%(module)s',
'line': '%(lineno)d',
'tms': '%(created)d',
'trace': '%(exc_text)s',
"proc": '%(processName)s',
"message": "%(message).5s"
}
},
},
'handlers': {
'console': {
'level': 'INFO',
'class': 'logging.StreamHandler',
'formatter': 'verbose'
},
'file': {
'level': 'INFO',
'class': 'logging.handlers.TimedRotatingFileHandler',
'filename': 'celery_nohup',
'when': 'D', # this specifies the interval
'interval': 1, # defaults to 1, only necessary for other values
'formatter': 'verbose',
},
'fluentd': {
'level': 'INFO',
'class': 'fluent.handler.FluentHandler',
'formatter': 'fluent_fmt',
'tag': 'default.log',
'host': 'localhost',
'port': 24224,
'timeout':3.0,
'verbose': False,
"msgpack_kwargs":{'default' : str}
},

},
'loggers': {
'django': {
'handlers': ['console'],
'level': "INFO",
'propagate': True,
},
'celery': {
'handlers': ['console', 'fluentd'],
'level': "INFO",
'propagate': True,
},
'ap_scheduler': {
'handlers': ['console', 'fluentd'],
'level': "INFO",
'propagate': True,
},
},
}
```

**expected output: message truncated to 5 characters**
fluentd_1 | 2022-06-02 18:58:43.000000000 +0000 default.log: `{"lvl":"INFO","host":"E7440","mod":"mingle","line":"40","tms":"1654196323","trace":"None","proc":"MainProcess","message":"mingl"}`

**obtained output**
fluentd_1 | 2022-06-02 18:58:43.000000000 +0000 default.log: `{"lvl":"INFO","host":"E7440","mod":"mingle","line":"40","tms":"1654196323","trace":"None","proc":"MainProcess","message":"mingle: searching for neighbors"}`