add shutdown method on MetricExporter interface (#475)

* feat: add shutdown method on MetricExporter interface

* fix: update JSDoc comment

Author: mayurkale22

packages/opentelemetry-metrics/src/export/types.ts

@@ -342,9 +342,12 @@ export interface MetricProducer {
  * Base interface that represents a metric exporter
  */
 export interface MetricExporter {
-  /** Exports the list of a given metric */
+  /** Exports the list of a given {@link ReadableMetric} */
   export(
     metrics: ReadableMetric[],
     resultCallback: (result: ExportResult) => void
   ): void;
+
+  /** Stops the exporter. */
+  shutdown(): void;
 }