[FLINK-10715] Suppress ConcurrnetModificationException in Slf4jReporter

This commit suppress the ConcurrentModificationException thrown by the Slf4jReporter
report method. This exception occurs when a metric is added/removed while this report is reporting.
Suppressing this (somewhat expected exception) is needed by our e2e framework to reduce some
false-positive e2e test failures. This does not solves the root issue, which is being tracked in a different issue FLINK-10035.

This closes apache#7008.

flink-metrics/flink-metrics-slf4j/src/main/java/org/apache/flink/metrics/slf4j/Slf4jReporter.java

@@ -33,6 +33,7 @@
 import org.slf4j.Logger;
 import org.slf4j.LoggerFactory;
 
+import java.util.ConcurrentModificationException;
 import java.util.Map;
 
 /**
@@ -75,6 +76,16 @@ public void close() {
 
  @Override
  public void report() {
+ try {
+ tryReport();
+ }
+ catch (ConcurrentModificationException ignored) {
+ // at tryReport() we don't synchronize while iterating over the various maps which might cause a
+ // ConcurrentModificationException to be thrown, if concurrently a metric is being added or removed.
+ }
+ }
+
+ private void tryReport() {
  // initialize with previous size to avoid repeated resizing of backing array
  // pad the size to allow deviations in the final string, for example due to different double value representations
  StringBuilder builder = new StringBuilder((int) (previousSize * 1.1));