Log4j2Metrics inaccurate with async appender

[ksambhav]

io.micrometer.core.instrument.binder.logging.Log4j2Metrics reports inaccurate metrics in case of async appender.
The decision to increment the counter is done by method :-

        @Override
        public Result filter(LogEvent event) {

            if (!isAsyncLogger || isAsyncLoggerAndEndOfBatch(event)) {
                incrementCounter(event); // always increments the counter by 1
            }

            return Result.NEUTRAL;
        }

When isAsyncLoggerAndEndOfBatch(event) is true, counter should get incremented by "batch" size. (Not sure if its possible to get it from LogEvent parameter.

Environment
PROD

• Micrometer version : 1.11.1
• Micrometer registry : Any. I tried with AzureMetricRegistry and SimpleRegistry
• OS: Linux
• Java version: 17

To Reproduce
How to reproduce the bug:
Unzip and run attached Spring Boot application. It prints 2000 error level logs in burst every 30 secs.
Counter can be accessed using http://localhost:8080/actuator/metrics/log4j2.events

Repeat again with sync appender by change <AsyncRoot> to simply <Root> in log4j2.xml. This time counter value will be correct this time.

Expected behavior
Counter should report accurate metrics in case of async appender

Additional context
https://stackoverflow.com/questions/76660558/log4j2-metrics-with-micrometer
Archive.zip

[shakuzen]

It sounds like this is a duplicate of #2176. Let me know if you disagree and why. We can reopen if it is a separate issue.

[ksambhav]

If the fix for #2176 will also cover case when counter is increment by the "batch size" instead just 1, we can keep this one closed. In the end we just want accurate count anyways.