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

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;
 }