Code snippet for loading SpectorJS in WebGL page. Copy this code to the development console of a web page with WebGL content. Make sure that you are posting it in the right context(if the canvas is inside an iframe).

load_spectorjs_in_dev_console.js

var newScript = document.createElement("script");
newScript.onload = function() { 
  var spector = new SPECTOR.Spector();
  spector.displayUI();
};
document.head.appendChild(newScript);
newScript.src = "https://spectorcdn.babylonjs.com/spector.bundle.js";

I think this doesn't work if the page you're attempting to using this on isn't setting appropriate CORS headers.

Sometimes the SpectorJS CDN is down, alternative URL can be https://unpkg.com/spectorjs/dist/spector.bundle.js