ZOOKEEPER-3694: Use Map computeIfAbsent in AvgMinMaxCounterSet Class

Author: David Mollitor <dmollitor@apache.org>

Reviewers: eolivelli@apache.org, maoling199210191@sina.com, fangmin@apache.org

Closes #1218 from belugabehr/ZOOKEEPER-3694

zookeeper-server/src/main/java/org/apache/zookeeper/server/metric/AvgMinMaxCounterSet.java

@@ -39,13 +39,7 @@ public class AvgMinMaxCounterSet extends Metric implements SummarySet {
     }
 
     private AvgMinMaxCounter getCounterForKey(String key) {
-        AvgMinMaxCounter counter = counters.get(key);
-        if (counter == null) {
-            counters.putIfAbsent(key, new AvgMinMaxCounter(key + "_" + name));
-            counter = counters.get(key);
-        }
-
-        return counter;
+        return counters.computeIfAbsent(key, k-> new AvgMinMaxCounter(k + "_" + name));
     }
 
     public void addDataPoint(String key, long value) {