chore: adding metric observable to be able to support async upda… (op…

…en-telemetry#964)

* chore: adding metric observable to be able to support async update

* chore: reviews

* chore: reviews

Co-authored-by: Daniel Dyla <dyladan@users.noreply.github.com>

api/src/index.ts

@@ -26,6 +26,7 @@ export * from './metrics/BoundInstrument';
 export * from './metrics/Meter';
 export * from './metrics/MeterProvider';
 export * from './metrics/Metric';
+export * from './metrics/MetricObservable';
 export * from './metrics/NoopMeter';
 export * from './metrics/NoopMeterProvider';
 export * from './metrics/ObserverResult';

api/src/metrics/MetricObservable.ts

@@ -0,0 +1,36 @@
+/*!
+ * Copyright 2020, OpenTelemetry Authors
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+/**
+ * Metric Observable class to handle asynchronous metrics
+ */
+export interface MetricObservable {
+  /**
+   * Sets the next value for observable metric
+   * @param value
+   */
+  next: (value: number) => void;
+  /**
+   * Subscribes for every value change
+   * @param callback
+   */
+  subscribe: (callback: (value: number) => void) => void;
+  /**
+   * Removes the subscriber
+   * @param [callback]
+   */
+  unsubscribe: (callback?: (value: number) => void) => void;
+}

api/src/metrics/ObserverResult.ts

@@ -15,11 +15,11 @@
  */
 
 import { Labels } from './Metric';
+import { MetricObservable } from './MetricObservable';
 
 /**
  * Interface that is being used in function setCallback for Observer Metric
  */
 export interface ObserverResult {
-  observers: Map<Labels, Function>;
-  observe(callback: Function, labels: Labels): void;
+  observe(callback: Function | MetricObservable, labels: Labels): void;
 }