feat[react-devtools-extensions/logging]: initialize session id on the client for logging (#27517)

This code is executed once React DevTools panels are mounted, basically
when user opens browser's DevTools. Based on this fact, session in this
case is defined by browser's DevTools session, while they are open. A
session can involve debugging multiple React pages. `crypto.randomUUID`
is used to generate random user id.

Corresponding logger config changes -
[D50267871](https://www.internalfb.com/diff/D50267871).

packages/react-devtools-shared/src/registerDevToolsEventLogger.js

@@ -12,7 +12,8 @@ import type {LoggerEvent} from 'react-devtools-shared/src/Logger';
 import {registerEventLogger} from 'react-devtools-shared/src/Logger';
 import {enableLogger} from 'react-devtools-feature-flags';
 
-let loggingIFrame = null;
+let currentLoggingIFrame = null;
+let currentSessionId = null;
 let missedEvents: Array<LoggerEvent> = [];
 
 type LoggerContext = {
@@ -21,32 +22,27 @@ type LoggerContext = {
 
 export function registerDevToolsEventLogger(
   surface: string,
-  fetchAdditionalContext: ?() =>
-    | LoggerContext
-    | ?(() => Promise<LoggerContext>),
+  fetchAdditionalContext?:
+    | (() => LoggerContext)
+    | (() => Promise<LoggerContext>),
 ): void {
   async function logEvent(event: LoggerEvent) {
     if (enableLogger) {
-      if (loggingIFrame != null) {
-        let metadata = null;
-        if (event.metadata != null) {
-          metadata = event.metadata;
-          // $FlowFixMe[cannot-write]: metadata is not writable and nullable
-          // $FlowFixMe[prop-missing]
-          delete event.metadata;
-        }
-        loggingIFrame.contentWindow.postMessage(
+      if (currentLoggingIFrame != null && currentSessionId != null) {
+        const {metadata, ...eventWithoutMetadata} = event;
+        const additionalContext: LoggerContext | {} =
+          fetchAdditionalContext != null ? await fetchAdditionalContext() : {};
+
+        currentLoggingIFrame?.contentWindow?.postMessage(
           {
             source: 'react-devtools-logging',
-            event: event,
+            event: eventWithoutMetadata,
             context: {
+              ...additionalContext,
+              metadata: metadata != null ? JSON.stringify(metadata) : '',
+              session_id: currentSessionId,
               surface,
               version: process.env.DEVTOOLS_VERSION,
-              metadata: metadata !== null ? JSON.stringify(metadata) : '',
-              ...(fetchAdditionalContext != null
-                ? // $FlowFixMe[not-an-object]
-                  await fetchAdditionalContext()
-                : {}),
             },
           },
           '*',
@@ -58,11 +54,8 @@ export function registerDevToolsEventLogger(
   }
 
   function handleLoggingIFrameLoaded(iframe: HTMLIFrameElement) {
-    if (loggingIFrame != null) {
-      return;
-    }
+    currentLoggingIFrame = iframe;
 
-    loggingIFrame = iframe;
     if (missedEvents.length > 0) {
       missedEvents.forEach(event => logEvent(event));
       missedEvents = [];
@@ -74,18 +67,21 @@ export function registerDevToolsEventLogger(
   if (enableLogger) {
     const loggingUrl = process.env.LOGGING_URL;
     const body = document.body;
+
     if (
       typeof loggingUrl === 'string' &&
       loggingUrl.length > 0 &&
-      body != null
+      body != null &&
+      currentLoggingIFrame == null
     ) {
       registerEventLogger(logEvent);
+      currentSessionId = window.crypto.randomUUID();
 
       const iframe = document.createElement('iframe');
+
+      iframe.onload = () => handleLoggingIFrameLoaded(iframe);
       iframe.src = loggingUrl;
-      iframe.onload = function (...args) {
-        handleLoggingIFrameLoaded(iframe);
-      };
+
       body.appendChild(iframe);
     }
   }