Make Canvas expressions use socket.io polling only (#29647)

This gets Canvas expressions to behave consistently across environments, and is the first step in migrating expressions to a different transport layer.

packages/kbn-interpreter/src/public/socket.js

@@ -26,7 +26,7 @@ export async function createSocket(basePath, functionsRegistry) {
   return new Promise((resolve, reject) => {
     const socket = io({
       path: `${basePath}/socket.io`,
-      transports: ['polling', 'websocket'],
+      transports: ['polling'],
       transportOptions: {
         polling: {
           extraHeaders: {

src/legacy/core_plugins/interpreter/server/routes/socket.js

@@ -46,7 +46,10 @@ export function socketApi(server) {
     handler: () => 'pong',
   });
 
-  const io = socket(server.listener, { path: '/socket.io' });
+  const io = socket(server.listener, {
+    path: '/socket.io',
+    transports: ['polling'],
+  });
 
   io.on('connection', async socket => {
     // 'request' is the modified hapi request object