Remove isEndOfBatch check from Log4j2Metrics

In case of async loggers, the MetricsFilter#filter method is/was called
multiple times. Because of this, a check was introduced in gh-2183
assuming that if the event has the isEndOfBatch flag set to true, that
is the last filter method call for that event.
Unfortunately, it turned out that this approach did not work, since it
did not just filter out the unwanted multiple calls on one event but it
also filtered out all the filter method calls on events that were not at
the end of the async batch. So Log4j2Metrics counted batches of events,
not the individual events.
Fortunately multiple filter invocations was fixed in Log4j2,
see apache/logging-log4j2#1550 and apache/logging-log4j2#1552.
Since now there will be only one filter method call,
the check introduced in gh-2183 can and should be removed (the call to
the filter method is before the isEndOfBatch flag is set so
the flag will always return false).

Closes gh-2176
See gh-2183
See gh-4253

micrometer-core/src/main/java/io/micrometer/core/instrument/binder/logging/Log4j2Metrics.java

@@ -26,7 +26,6 @@
 import org.apache.logging.log4j.core.Filter;
 import org.apache.logging.log4j.core.LogEvent;
 import org.apache.logging.log4j.core.LoggerContext;
-import org.apache.logging.log4j.core.async.AsyncLoggerConfig;
 import org.apache.logging.log4j.core.config.Configuration;
 import org.apache.logging.log4j.core.config.LoggerConfig;
 import org.apache.logging.log4j.core.filter.AbstractFilter;
@@ -39,7 +38,12 @@
 import static java.util.Collections.emptyList;
 
 /**
- * {@link MeterBinder} for Apache Log4j 2.
+ * {@link MeterBinder} for Apache Log4j 2. Please use at least 2.21.0 since there was a
+ * bug in earlier versions that prevented Micrometer to increment its counters correctly.
+ * See:
+ * <a href="https://github.com/apache/logging-log4j2/issues/1550">logging-log4j2#1550</a>
+ * See: <a href=
+ * "https://github.com/micrometer-metrics/micrometer/issues/2176">micrometer#2176</a>
  *
  * @author Steven Sheehy
  * @author Johnny Lim
@@ -55,7 +59,7 @@ public class Log4j2Metrics implements MeterBinder, AutoCloseable {
 
     private final LoggerContext loggerContext;
 
-    private List<MetricsFilter> metricsFilters = new ArrayList<>();
+    private final List<MetricsFilter> metricsFilters = new ArrayList<>();
 
     public Log4j2Metrics() {
         this(emptyList());
@@ -75,7 +79,7 @@ public void bindTo(MeterRegistry registry) {
 
         Configuration configuration = loggerContext.getConfiguration();
         LoggerConfig rootLoggerConfig = configuration.getRootLogger();
-        rootLoggerConfig.addFilter(createMetricsFilterAndStart(registry, rootLoggerConfig));
+        rootLoggerConfig.addFilter(createMetricsFilterAndStart(registry));
 
         loggerContext.getConfiguration()
             .getLoggers()
@@ -97,14 +101,14 @@ public void bindTo(MeterRegistry registry) {
                 if (logFilter instanceof MetricsFilter) {
                     return;
                 }
-                loggerConfig.addFilter(createMetricsFilterAndStart(registry, loggerConfig));
+                loggerConfig.addFilter(createMetricsFilterAndStart(registry));
             });
 
         loggerContext.updateLoggers(configuration);
     }
 
-    private MetricsFilter createMetricsFilterAndStart(MeterRegistry registry, LoggerConfig loggerConfig) {
-        MetricsFilter metricsFilter = new MetricsFilter(registry, tags, loggerConfig instanceof AsyncLoggerConfig);
+    private MetricsFilter createMetricsFilterAndStart(MeterRegistry registry) {
+        MetricsFilter metricsFilter = new MetricsFilter(registry, tags);
         metricsFilter.start();
         metricsFilters.add(metricsFilter);
         return metricsFilter;
@@ -135,7 +139,7 @@ public void close() {
 
     @NonNullApi
     @NonNullFields
-    class MetricsFilter extends AbstractFilter {
+    static class MetricsFilter extends AbstractFilter {
 
         private final Counter fatalCounter;
 
@@ -149,10 +153,7 @@ class MetricsFilter extends AbstractFilter {
 
         private final Counter traceCounter;
 
-        private final boolean isAsyncLogger;
-
-        MetricsFilter(MeterRegistry registry, Iterable<Tag> tags, boolean isAsyncLogger) {
-            this.isAsyncLogger = isAsyncLogger;
+        MetricsFilter(MeterRegistry registry, Iterable<Tag> tags) {
             fatalCounter = Counter.builder(METER_NAME)
                 .tags(tags)
                 .tags("level", "fatal")
@@ -198,18 +199,10 @@ class MetricsFilter extends AbstractFilter {
 
         @Override
         public Result filter(LogEvent event) {
-
-            if (!isAsyncLogger || isAsyncLoggerAndEndOfBatch(event)) {
-                incrementCounter(event);
-            }
-
+            incrementCounter(event);
             return Result.NEUTRAL;
         }
 
-        private boolean isAsyncLoggerAndEndOfBatch(LogEvent event) {
-            return isAsyncLogger && event.isEndOfBatch();
-        }
-
         private void incrementCounter(LogEvent event) {
             switch (event.getLevel().getStandardLevel()) {
                 case FATAL:

micrometer-core/src/test/java/io/micrometer/core/instrument/binder/logging/Log4j2MetricsTest.java

@@ -174,8 +174,10 @@ void asyncLogShouldNotBeDuplicated() throws IOException {
 
         assertThat(registry.get("log4j2.events").tags("level", "info").counter().count()).isEqualTo(0);
         logger.info("Hello, world!");
+        logger.info("Hello, world!");
+        logger.info("Hello, world!");
         await().atMost(Duration.ofSeconds(1))
-            .until(() -> registry.get("log4j2.events").tags("level", "info").counter().count() == 1);
+            .until(() -> registry.get("log4j2.events").tags("level", "info").counter().count() == 3);
     }
 
 }