Monitor threw ConcurrentModificationException when loading Tablet Server page

[austin007008]

Describe the bug
When loading the tablet server page of the monitor, I noticed that the monitor threw a "ConcurrentModificationException" (seen on the "recent logs" page).

Versions (OS, Maven, Java, and others, as appropriate):
accumulo 2.1.3

To Reproduce
Steps to reproduce the behavior (or a link to an example repository that reproduces the problem):
Can not reproduce

Expected behavior
No ConcurrentModificationException thrown

Additional context
caused by: java.util.ConcurrentmodificationException
at java.util.HashMap$HashIterator.nextNode(HashMap.java:1511)
at java.util.HashMap$HashIterator.next(HashMap.java:1544)
at java.util.HashMap$HashIterator.next(HashMap.java:1542)
at org.apache.accumulo.monitor.Monitor$EventCounter.calculateRate(monitor.java:23)
at org.apache.accumulo.monitor.Monitor.getLookupRate(Monitor.java:977)
at org.apache.accumulo.monitor.rest.tservers.TabletServerInformation.updateTabletServerInfo(TabletServerinformation.java:162)
at org.apache.accumulo.monitor.rest.tservers.TabletServerInformation.(TabletServerInformation:101)
at org.apache.accumulo.monitor.rest.tservers.TabletServer.(TabletServer.java:43)
at org.apache.accumulo.monitor.rest.tservers.TabletServerResource.getTserverSummary(TabletServerResource.java:88)