Return an interface when registering metric callbacks. (#4022)

* Return an interface when registring metrics.

* Return type-specific

* untypo

Author: anuraaga

api/all/src/main/java/io/opentelemetry/api/metrics/DefaultMeter.java

@@ -86,6 +86,9 @@ public void add(double value) {}
   }
 
   private static class NoopLongCounterBuilder implements LongCounterBuilder {
+
+    private static final ObservableLongCounter NOOP = new ObservableLongCounter() {};
+
     @Override
     public LongCounterBuilder setDescription(String description) {
       return this;
@@ -107,10 +110,14 @@ public LongCounter build() {
     }
 
     @Override
-    public void buildWithCallback(Consumer<ObservableLongMeasurement> callback) {}
+    public ObservableLongCounter buildWithCallback(Consumer<ObservableLongMeasurement> callback) {
+      return NOOP;
+    }
   }
 
   private static class NoopDoubleCounterBuilder implements DoubleCounterBuilder {
+    private static final ObservableDoubleCounter NOOP = new ObservableDoubleCounter() {};
+
     @Override
     public DoubleCounterBuilder setDescription(String description) {
       return this;
@@ -127,7 +134,10 @@ public DoubleCounter build() {
     }
 
     @Override
-    public void buildWithCallback(Consumer<ObservableDoubleMeasurement> callback) {}
+    public ObservableDoubleCounter buildWithCallback(
+        Consumer<ObservableDoubleMeasurement> callback) {
+      return NOOP;
+    }
   }
 
   private static class NoopLongUpDownCounter implements LongUpDownCounter {
@@ -153,6 +163,8 @@ public void add(double value) {}
   }
 
   private static class NoopLongUpDownCounterBuilder implements LongUpDownCounterBuilder {
+    private static final ObservableLongUpDownCounter NOOP = new ObservableLongUpDownCounter() {};
+
     @Override
     public LongUpDownCounterBuilder setDescription(String description) {
       return this;
@@ -174,10 +186,16 @@ public LongUpDownCounter build() {
     }
 
     @Override
-    public void buildWithCallback(Consumer<ObservableLongMeasurement> callback) {}
+    public ObservableLongUpDownCounter buildWithCallback(
+        Consumer<ObservableLongMeasurement> callback) {
+      return NOOP;
+    }
   }
 
   private static class NoopDoubleUpDownCounterBuilder implements DoubleUpDownCounterBuilder {
+    private static final ObservableDoubleUpDownCounter NOOP =
+        new ObservableDoubleUpDownCounter() {};
+
     @Override
     public DoubleUpDownCounterBuilder setDescription(String description) {
       return this;
@@ -194,7 +212,10 @@ public DoubleUpDownCounter build() {
     }
 
     @Override
-    public void buildWithCallback(Consumer<ObservableDoubleMeasurement> callback) {}
+    public ObservableDoubleUpDownCounter buildWithCallback(
+        Consumer<ObservableDoubleMeasurement> callback) {
+      return NOOP;
+    }
   }
 
   public static class NoopDoubleHistogram implements DoubleHistogram {
@@ -259,6 +280,8 @@ public LongHistogram build() {
   }
 
   public static class NoopDoubleObservableInstrumentBuilder implements DoubleGaugeBuilder {
+    private static final ObservableDoubleGauge NOOP = new ObservableDoubleGauge() {};
+
     @Override
     public DoubleGaugeBuilder setDescription(String description) {
       return this;
@@ -275,10 +298,14 @@ public LongGaugeBuilder ofLongs() {
     }
 
     @Override
-    public void buildWithCallback(Consumer<ObservableDoubleMeasurement> callback) {}
+    public ObservableDoubleGauge buildWithCallback(Consumer<ObservableDoubleMeasurement> callback) {
+      return NOOP;
+    }
   }
 
   public static class NoopLongObservableInstrumentBuilder implements LongGaugeBuilder {
+    private static final ObservableLongGauge NOOP = new ObservableLongGauge() {};
+
     @Override
     public LongGaugeBuilder setDescription(String description) {
       return this;
@@ -290,6 +317,8 @@ public LongGaugeBuilder setUnit(String unit) {
     }
 
     @Override
-    public void buildWithCallback(Consumer<ObservableLongMeasurement> callback) {}
+    public ObservableLongGauge buildWithCallback(Consumer<ObservableLongMeasurement> callback) {
+      return NOOP;
+    }
   }
 }

api/all/src/main/java/io/opentelemetry/api/metrics/DoubleCounterBuilder.java

@@ -47,5 +47,5 @@ public interface DoubleCounterBuilder {
    *
    * @param callback A state-capturing callback used to observe values on-demand.
    */
-  void buildWithCallback(Consumer<ObservableDoubleMeasurement> callback);
+  ObservableDoubleCounter buildWithCallback(Consumer<ObservableDoubleMeasurement> callback);
 }

api/all/src/main/java/io/opentelemetry/api/metrics/DoubleGaugeBuilder.java

@@ -42,5 +42,5 @@ public interface DoubleGaugeBuilder {
    *
    * @param callback A state-capturing callback used to observe values on-demand.
    */
-  void buildWithCallback(Consumer<ObservableDoubleMeasurement> callback);
+  ObservableDoubleGauge buildWithCallback(Consumer<ObservableDoubleMeasurement> callback);
 }

api/all/src/main/java/io/opentelemetry/api/metrics/DoubleUpDownCounterBuilder.java

@@ -46,5 +46,5 @@ public interface DoubleUpDownCounterBuilder {
    *
    * @param callback A state-capturing callback used to observe values on-demand.
    */
-  void buildWithCallback(Consumer<ObservableDoubleMeasurement> callback);
+  ObservableDoubleUpDownCounter buildWithCallback(Consumer<ObservableDoubleMeasurement> callback);
 }

api/all/src/main/java/io/opentelemetry/api/metrics/LongCounterBuilder.java

@@ -51,5 +51,5 @@ public interface LongCounterBuilder {
    *
    * @param callback A state-capturing callback used to observe values on-demand.
    */
-  void buildWithCallback(Consumer<ObservableLongMeasurement> callback);
+  ObservableLongCounter buildWithCallback(Consumer<ObservableLongMeasurement> callback);
 }

api/all/src/main/java/io/opentelemetry/api/metrics/LongGaugeBuilder.java

@@ -39,5 +39,5 @@ public interface LongGaugeBuilder {
    *
    * @param callback A state-capturing callback used to observe values on-demand.
    */
-  void buildWithCallback(Consumer<ObservableLongMeasurement> callback);
+  ObservableLongGauge buildWithCallback(Consumer<ObservableLongMeasurement> callback);
 }

api/all/src/main/java/io/opentelemetry/api/metrics/LongUpDownCounterBuilder.java

@@ -49,5 +49,5 @@ public interface LongUpDownCounterBuilder {
    *
    * @param callback A state-capturing callback used to observe values on-demand.
    */
-  void buildWithCallback(Consumer<ObservableLongMeasurement> callback);
+  ObservableLongUpDownCounter buildWithCallback(Consumer<ObservableLongMeasurement> callback);
 }

api/all/src/main/java/io/opentelemetry/api/metrics/ObservableDoubleCounter.java

@@ -0,0 +1,17 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.api.metrics;
+
+import java.util.function.Consumer;
+
+/**
+ * A reference to an observable metric registered with {@link
+ * DoubleCounterBuilder#buildWithCallback(Consumer)}.
+ *
+ * <p>This interface currently has no methods but may be extended in the future with functionality
+ * such as canceling the observable.
+ */
+public interface ObservableDoubleCounter {}

api/all/src/main/java/io/opentelemetry/api/metrics/ObservableDoubleGauge.java

@@ -0,0 +1,17 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.api.metrics;
+
+import java.util.function.Consumer;
+
+/**
+ * A reference to an observable metric registered with {@link
+ * DoubleGaugeBuilder#buildWithCallback(Consumer)}.
+ *
+ * <p>This interface currently has no methods but may be extended in the future with functionality
+ * such as canceling the observable.
+ */
+public interface ObservableDoubleGauge {}

api/all/src/main/java/io/opentelemetry/api/metrics/ObservableDoubleUpDownCounter.java

@@ -0,0 +1,17 @@
+/*
+ * Copyright The OpenTelemetry Authors
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+package io.opentelemetry.api.metrics;
+
+import java.util.function.Consumer;
+
+/**
+ * A reference to an observable metric registered with {@link
+ * DoubleUpDownCounterBuilder#buildWithCallback(Consumer)}.
+ *
+ * <p>This interface currently has no methods but may be extended in the future with functionality
+ * such as canceling the observable.
+ */
+public interface ObservableDoubleUpDownCounter {}