Pull custom RUM code into its own module

It's a bit surprising to have a bunch of our code tacked on to the end
of a vendored file. Our docs say the lux-measurer file "shouldn't ever
change", but that feels like a bit of a shaky assumption.

In any case, it feels much cleaner to have our code in its own file,
instead of muddling it with vendored code.

I've use script type=module for two reasons:

1) Like the design system, we can use it as a check that we're in a
   modern browser, which means we can use const / arrow functions and
   functions like `.some()` which might not be present in old browsers.
   For the same reason, we can simplify a lot of the checks around whether
   the performance API will be there, and whether it will work correctly.
2) It defers the execution of the script, so we'll automatically wait
   for the HTML to be fully parsed before our code runs. Although we're
   not interacting with the HTML, I think this should more or less
   guarantee that (a) the nextHopProtocol timing property is available
   and (b) the LUX global will already have been defined by lux-measurer.js
   Just in case (b) isn't always true, I've checked that LUX is defined
   before attempting to use it.

I'm also removing the try / catch code - it doesn't seem like it's doing
anything useful even if there is an error, and running as a module the
code shouldn't have any reference errors / surprisingly undefined APIs.

app/assets/javascripts/govuk_publishing_components/rum-custom-data.js

@@ -0,0 +1,12 @@
+(function () {
+  /* global LUX, performance */
+  if (typeof LUX === 'undefined') { return }
+
+  const navigationPerformance = performance.getEntriesByType('navigation')[0]
+  if (!navigationPerformance) { return }
+
+  // As per RFC 147[1], this adds in monitoring of the type of HTTP protocol that
+  // is used when a browser loads a page.
+  // [1]: https://github.com/alphagov/govuk-rfcs/pull/148
+  LUX.addData('http-protocol', navigationPerformance.nextHopProtocol)
+}())

app/assets/javascripts/govuk_publishing_components/vendor/lux/lux-measurer.js

@@ -157,40 +157,3 @@ if (
     LongTaskObserver.observe({ type: ["longtask"] });
   } catch (e) {}
 }
-
-// As per RFC 147[1], this adds in monitoring of the type of HTTP protocol that
-// is used when a browser loads a page.
-//
-// The User Timing API (aka window.performance) is used to record the data - to
-// avoid the use of this from breaking the JavaScript for the small number of
-// browsers that don't support it, it's been wrapped in a try/catch block plus a
-// couple of checks to prevent "is not defined" errors.
-//
-// Because the `nextHopProtocol` isn't immediately available - it seems to need
-// a request to be made before it's populated - we need to wait for the
-// `DOMContentReady` event before we can see what the HTTP version is.
-//
-// [1]: https://github.com/alphagov/govuk-rfcs/pull/148
-
-var measureHTTPProtocol = function () {
-  var getEntriesByType = performance.getEntriesByType('navigation')
-
-  if (typeof getEntriesByType !== 'undefined' && getEntriesByType.length > 0) {
-    var httpProtocol = getEntriesByType[0].nextHopProtocol
-    LUX.addData("http-protocol", httpProtocol)
-  }
-}
-
-try {
-  if (typeof performance !== 'undefined' && typeof performance.getEntriesByType !== 'undefined') {
-    if (document.readyState === 'complete') {
-      measureHTTPProtocol()
-    } else {
-      window.addEventListener('load', function() {
-        measureHTTPProtocol()
-      })
-    }
-  }
-} catch (e) {
-  console.error('Error in LUX reporting the HTTP protocol (%s):', window.location, e)
-}

app/views/govuk_publishing_components/components/_layout_for_public.html.erb

@@ -65,6 +65,7 @@
     <title><%= title %></title>
 
     <%= javascript_include_tag "govuk_publishing_components/vendor/lux/lux-measurer", { async: true } %>
+    <%= javascript_include_tag "govuk_publishing_components/rum-custom-data", type: "module" %>
     <%= javascript_include_tag "govuk_publishing_components/rum-loader",
       {
         async: true,

spec/support/jasmine-browser.json

@@ -7,6 +7,7 @@
     "govuk_publishing_components/analytics-*.js",
     "govuk_publishing_components/load-analytics-*.js",
     "govuk_publishing_components/vendor/lux/lux-measurer-*.js",
+    "govuk_publishing_components/rum-custom-data-*.js",
     "component_guide/accessibility-test-*.js",
     "component_guide/filter-components-*.js",
     "component_guide/audit-filter-*.js"