Add SerializedEvent type to JsonReporter

Summary:
Changelog: [Internal]

Adds a `SerializedEvent<T>` helper type that describes how a `Logger` event of type `T` would be serialized by Metro's `JsonReporter` class.

Reviewed By: GijsWeterings

Differential Revision: D55649234

fbshipit-source-id: 757ae1d4c6f99c7e86fb92e4074e0c1d70001702

packages/metro/src/lib/JsonReporter.js

@@ -13,6 +13,18 @@
 
 import type {Writable} from 'stream';
 
+export type SerializedEvent<TEvent: {[string]: any, ...}> = TEvent extends {
+  error: Error,
+  ...
+}
+  ? {
+      ...Omit<TEvent, 'error'>,
+      message: string,
+      stack: string,
+      ...
+    }
+  : TEvent;
+
 class JsonReporter<TEvent: {[string]: any, ...}> {
   _stream: Writable;