[FLINK-4829] snapshot accumulators on a best-effort basis

Heartbeats should not fail when accumulators could not be snapshotted. Instead,
we should simply skip the reporting of the failed accumulator. Eventually, the
accumulator will be reported; at the latest, when the job finishes.

This closes apache#2649

flink-runtime/src/main/scala/org/apache/flink/runtime/taskmanager/TaskManager.scala

@@ -35,6 +35,7 @@ import com.codahale.metrics.jvm.{BufferPoolMetricSet, GarbageCollectorMetricSet,
 import com.codahale.metrics.{Gauge, MetricFilter, MetricRegistry}
 import com.fasterxml.jackson.databind.ObjectMapper
 import grizzled.slf4j.Logger
+import org.apache.commons.lang3.exception.ExceptionUtils
 import org.apache.flink.configuration._
 import org.apache.flink.core.fs.FileSystem
 import org.apache.flink.core.memory.{HeapMemorySegment, HybridMemorySegment, MemorySegmentFactory, MemoryType}
@@ -1335,9 +1336,15 @@ class TaskManager(
 
       runningTasks.asScala foreach {
         case (execID, task) =>
-          val registry = task.getAccumulatorRegistry
-          val accumulators = registry.getSnapshot
-          accumulatorEvents.append(accumulators)
+          try {
+            val registry = task.getAccumulatorRegistry
+            val accumulators = registry.getSnapshot
+            accumulatorEvents.append(accumulators)
+          } catch {
+            case e: Exception =>
+              log.warn("Failed to take accumulator snapshot for task {}.",
+                execID, ExceptionUtils.getRootCause(e))
+          }
       }
 
        currentJobManager foreach {