Record the event timestamp before dispatching

Recording the timestamp in a dispatched function
could potentially delay its recording and provide
inaccurate timestamps.

Author: Dexterp37

glean/src/core/metrics/types/event.ts

@@ -33,13 +33,13 @@ class EventMetricType extends MetricType {
    *        The maximum length for values is 100 bytes.
    */
   record(extra?: ExtraMap): void {
+    const timestamp = getMonotonicNow();
+
     Context.dispatcher.launch(async () => {
       if (!this.shouldRecord(Context.uploadEnabled)) {
         return;
       }
 
-      const timestamp = getMonotonicNow();
-
       // Truncate the extra keys, if needed.
       let truncatedExtra: ExtraMap | undefined = undefined;
       if (extra && this.allowedExtraKeys) {