Log4j2Metrics creates more MetricsFilter instances than needed

[shakuzen]

Before, we were needlessly making multiple instances of MetricsFilter that were functionally equivalent. This reduces the MetricsFilter instances to one per registry to which a Log4j2Metrics instance is bound. This will reduce allocations and memory usage slightly.

[shakuzen]

Merging before review so we don't keep blocking #5810, but anyone feel free to leave review post merge and we can follow-up with anything that comes up.

[pativa]

@shakuzen
While reading the code while looking at #5810 I saw another issue in this fix (or rather, this does not fully support binding to multiple MeterRegistry). The issue is that it only works on rootLoggerConfig, for all other logger configs the second filter will not be added.

Here is a modified test from the test added in this PR that does not pass as the non-root logger configuration is not tracked for the second MeterRegistry:

    @Test
    void multipleRegistriesCanBeBound() {
        LoggerContext loggerContext = new LoggerContext("test");

        LoggerConfig loggerConfig = new LoggerConfig("com.test", Level.INFO, false);
        Configuration configuration = loggerContext.getConfiguration();
        configuration.getRootLogger().setLevel(Level.INFO);
        configuration.addLogger("com.test", loggerConfig);
        loggerContext.updateLoggers(configuration);

        MeterRegistry registry2 = new SimpleMeterRegistry();

        Log4j2Metrics log4j2Metrics = new Log4j2Metrics(emptyList(), loggerContext);
        log4j2Metrics.bindTo(registry);

        // verify root logger
        Logger logger = loggerContext.getLogger(Log4j2MetricsTest.class);
        logger.info("Hello, world!");
        assertThat(registry.get("log4j2.events").tags("level", "info").counter().count()).isEqualTo(1);

        // verify other logger
        Logger logger2 = loggerContext.getLogger("com.test");
        logger2.info("Using other logger than root logger");
        assertThat(registry.get("log4j2.events").tags("level", "info").counter().count()).isEqualTo(2);

        log4j2Metrics.bindTo(registry2);

        logger.info("Hello, world!");
        assertThat(registry.get("log4j2.events").tags("level", "info").counter().count()).isEqualTo(3);
        assertThat(registry2.get("log4j2.events").tags("level", "info").counter().count()).isEqualTo(1);

        logger2.info("Using other logger than root logger");
        assertThat(registry.get("log4j2.events").tags("level", "info").counter().count()).isEqualTo(4);
        // this final check does not pass as the log event is not properly counted
        assertThat(registry2.get("log4j2.events").tags("level", "info").counter().count()).isEqualTo(2);
    }

The issue is the instanceof checks in Log4j2Metrics, it needs to check the correct instance of MetricsFilter to properly support registering multiple MeterRegistry (which is a very odd case anyway).

[shakuzen]

@pativa thank you for taking a look and for providing a failing unit test. I'll take a closer look tomorrow.

[shakuzen]

Looking at the code, it seems this should have been a problem before as well. I don't know that binding to multiple registries was ever working properly so maybe it isn't a bug to fix, but the fix seems easy enough. I'll open a separate issue and use your test code.