Create and implement getSum

Author: charlesmyu

products/metrics/metrics-api/src/main/java/datadog/metrics/api/Histogram.java

@@ -5,6 +5,8 @@
 
 public interface Histogram {
 
+  double getSum();
+
   double getCount();
 
   boolean isEmpty();

products/metrics/metrics-api/src/main/java/datadog/metrics/api/NoOpHistogram.java

@@ -12,6 +12,11 @@ public double getCount() {
     return 0;
   }
 
+  @Override
+  public double getSum() {
+    return 0;
+  }
+
   @Override
   public boolean isEmpty() {
     return true;

products/metrics/metrics-lib/src/main/java/datadog/metrics/impl/DDSketchHistogram.java

@@ -12,16 +12,23 @@
 /** Wrapper around the DDSketch library so that it can be used in an instrumentation */
 public class DDSketchHistogram implements Histogram {
   private final DDSketch sketch;
+  private double sum;
 
   public DDSketchHistogram(DDSketch sketch) {
     this.sketch = sketch;
+    this.sum = 0;
   }
 
   @Override
   public double getCount() {
     return sketch.getCount();
   }
 
+  @Override
+  public double getSum() {
+    return sum;
+  }
+
   @Override
   public boolean isEmpty() {
     return sketch.isEmpty();
@@ -30,11 +37,13 @@ public boolean isEmpty() {
   @Override
   public void accept(double value) {
     sketch.accept(value);
+    sum += value;
   }
 
   @Override
   public void accept(double value, double count) {
     sketch.accept(value, count);
+    sum += value * count;
   }
 
   @Override