HADOOP-11447. Add a more meaningful toString method to SampleStat and MutableStat. (kasha)

(cherry picked from commit fe5c23b670)

hadoop-common-project/hadoop-common/CHANGES.txt

@@ -23,6 +23,9 @@ Release 2.8.0 - UNRELEASED
     HADOOP-11709. Time.NANOSECONDS_PER_MILLISECOND - use class-level final
     constant instead of method variable (Ajith S via ozawa)
 
+    HADOOP-11447. Add a more meaningful toString method to SampleStat and 
+    MutableStat. (kasha)
+
   OPTIMIZATIONS
 
   BUG FIXES

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/metrics2/lib/MutableStat.java

@@ -150,4 +150,8 @@ public class MutableStat extends MutableMetric {
     minMax.reset();
   }
 
+  @Override
+  public String toString() {
+    return lastStat().toString();
+  }
 }

hadoop-common-project/hadoop-common/src/main/java/org/apache/hadoop/metrics2/util/SampleStat.java

@@ -137,6 +137,19 @@ public class SampleStat {
     return minmax.max();
   }
 
+  @Override
+  public String toString() {
+    try {
+      return "Samples = " + numSamples() +
+          "  Min = " + min() +
+          "  Mean = " + mean() +
+          "  Std Dev = " + stddev() +
+          "  Max = " + max();
+    } catch (Throwable t) {
+      return super.toString();
+    }
+  }
+
   /**
    * Helper to keep running min/max
    */