logging should cache LogRecord.message

Bug report

Bug description:

logging currently does not cache LogRecord.message:
https://github.com/python/cpython/blob/0dcbc8385322ff51f7fc3e586027d880275df4fa/Lib/logging/__init__.py#L391-L401
https://github.com/python/cpython/blob/0dcbc8385322ff51f7fc3e586027d880275df4fa/Lib/logging/__init__.py#L711

Since logging caches other string-formatting related results such as LogRecord.asctime and LogRecord.exc_text, I would like to suggest:

1. LogRecord.getMessage should check self.message first. If it is not computed yet, it should compute the message and set this attribute before returning.
2. Formatter shouldn't set record.message. It only needs to call LogRecord.getMessage to get the message.

How to reproduce

import logging

class A:
    def __repr__(self):
        print('repr called')
        return '<A>'

logger = logging.getLogger()
logger.addHandler(logging.StreamHandler())
logger.addHandler(logging.StreamHandler())
logger.error('%r', A())

Expected output

repr called
<A>
<A>

Actual output

repr called
<A>
repr called
<A>

CPython versions tested on:

3.12

Operating systems tested on:

Linux

Linked PRs

• gh-134367

Read Lib/logging/init.py at the referenced LogRecord and Formatter locations, then run the reproduction to observe repeated repr calls. Review linked PR gh-134367 before starting. Done means the two-handler example calls repr once while both handlers produce the message.