Add Aggregation.LastValue and AggregationData.LastValueData to support Gauge (#1055)

* Add LastValue and LastValueData

* Support LastValue and LastValueData in impl

* Use Utils instead of Precondition

* Add LastValue and remove Mean from match() method.

* Support LastValue and LastValueData in exporters and zpages.

* Update MutableLastValue, add comments on why Mean is still supported.

Author: songy23

api/src/main/java/io/opencensus/stats/Aggregation.java

@@ -25,12 +25,13 @@
  * {@link Aggregation} is the process of combining a certain set of {@code MeasureValue}s for a
  * given {@code Measure} into an {@link AggregationData}.
  *
- * <p>{@link Aggregation} currently supports 3 types of basic aggregation:
+ * <p>{@link Aggregation} currently supports 4 types of basic aggregation:
  *
  * <ul>
  *   <li>Sum
  *   <li>Count
  *   <li>Distribution
+ *   <li>LastValue
  * </ul>
  *
  * <p>When creating a {@link View}, one {@link Aggregation} needs to be specified as how to
@@ -43,30 +44,16 @@ public abstract class Aggregation {
 
   private Aggregation() {}
 
-  /**
-   * Applies the given match function to the underlying data type.
-   *
-   * @since 0.8
-   * @deprecated in favor of {@link #match(Function, Function, Function, Function)}.
-   */
-  @Deprecated
-  public abstract <T> T match(
-      Function<? super Sum, T> p0,
-      Function<? super Count, T> p1,
-      Function<? super Mean, T> p2,
-      Function<? super Distribution, T> p3,
-      Function<? super Aggregation, T> defaultFunction);
-
   /**
    * Applies the given match function to the underlying data type.
    *
    * @since 0.13
    */
-  @SuppressWarnings("InconsistentOverloads")
   public abstract <T> T match(
       Function<? super Sum, T> p0,
       Function<? super Count, T> p1,
       Function<? super Distribution, T> p2,
+      Function<? super LastValue, T> p3,
       Function<? super Aggregation, T> defaultFunction);
 
   /**
@@ -92,21 +79,12 @@ public static Sum create() {
       return INSTANCE;
     }
 
-    @Override
-    public final <T> T match(
-        Function<? super Sum, T> p0,
-        Function<? super Count, T> p1,
-        Function<? super Mean, T> p2,
-        Function<? super Distribution, T> p3,
-        Function<? super Aggregation, T> defaultFunction) {
-      return p0.apply(this);
-    }
-
     @Override
     public final <T> T match(
         Function<? super Sum, T> p0,
         Function<? super Count, T> p1,
         Function<? super Distribution, T> p2,
+        Function<? super LastValue, T> p3,
         Function<? super Aggregation, T> defaultFunction) {
       return p0.apply(this);
     }
@@ -135,21 +113,12 @@ public static Count create() {
       return INSTANCE;
     }
 
-    @Override
-    public final <T> T match(
-        Function<? super Sum, T> p0,
-        Function<? super Count, T> p1,
-        Function<? super Mean, T> p2,
-        Function<? super Distribution, T> p3,
-        Function<? super Aggregation, T> defaultFunction) {
-      return p1.apply(this);
-    }
-
     @Override
     public final <T> T match(
         Function<? super Sum, T> p0,
         Function<? super Count, T> p1,
         Function<? super Distribution, T> p2,
+        Function<? super LastValue, T> p3,
         Function<? super Aggregation, T> defaultFunction) {
       return p1.apply(this);
     }
@@ -181,21 +150,12 @@ public static Mean create() {
       return INSTANCE;
     }
 
-    @Override
-    public final <T> T match(
-        Function<? super Sum, T> p0,
-        Function<? super Count, T> p1,
-        Function<? super Mean, T> p2,
-        Function<? super Distribution, T> p3,
-        Function<? super Aggregation, T> defaultFunction) {
-      return p2.apply(this);
-    }
-
     @Override
     public final <T> T match(
         Function<? super Sum, T> p0,
         Function<? super Count, T> p1,
         Function<? super Distribution, T> p2,
+        Function<? super LastValue, T> p3,
         Function<? super Aggregation, T> defaultFunction) {
       return defaultFunction.apply(this);
     }
@@ -236,19 +196,44 @@ public static Distribution create(BucketBoundaries bucketBoundaries) {
     public final <T> T match(
         Function<? super Sum, T> p0,
         Function<? super Count, T> p1,
-        Function<? super Mean, T> p2,
-        Function<? super Distribution, T> p3,
+        Function<? super Distribution, T> p2,
+        Function<? super LastValue, T> p3,
         Function<? super Aggregation, T> defaultFunction) {
-      return p3.apply(this);
+      return p2.apply(this);
+    }
+  }
+
+  /**
+   * Calculate the last value of aggregated {@code MeasureValue}s.
+   *
+   * @since 0.13
+   */
+  @Immutable
+  @AutoValue
+  public abstract static class LastValue extends Aggregation {
+
+    LastValue() {}
+
+    private static final LastValue INSTANCE = new AutoValue_Aggregation_LastValue();
+
+    /**
+     * Construct a {@code LastValue}.
+     *
+     * @return a new {@code LastValue}.
+     * @since 0.13
+     */
+    public static LastValue create() {
+      return INSTANCE;
     }
 
     @Override
     public final <T> T match(
         Function<? super Sum, T> p0,
         Function<? super Count, T> p1,
         Function<? super Distribution, T> p2,
+        Function<? super LastValue, T> p3,
         Function<? super Aggregation, T> defaultFunction) {
-      return p2.apply(this);
+      return p3.apply(this);
     }
   }
 }