docs(api-metrics): add notes on ObservableResult.observe (#2712)

Co-authored-by: Valentin Marchaud <contact@vmarchaud.fr>
Co-authored-by: Daniel Dyla <dyladan@users.noreply.github.com>

experimental/packages/opentelemetry-api-metrics/src/types/ObservableResult.ts

@@ -20,5 +20,13 @@ import { Attributes } from './Metric';
  * Interface that is being used in callback function for Observable Metric
  */
 export interface ObservableResult {
+  /**
+   * Observe a measurement of the value associated with the given attributes.
+   *
+   * @param value The value to be observed.
+   * @param attributes The attributes associated with the value. If more than
+   * one values associated with the same attributes values, SDK may pick the
+   * last one or simply drop the entire observable result.
+   */
   observe(value: number, attributes?: Attributes): void;
 }