Report cache status to RUM

We added the cache state to the Server-Timing header in alphagov/govuk-fastly#130

This uses the browser's performance API to check if there's a cacheHit /
cacheMiss value in the Server-Timing header, and if there is, report it
to RUM.

This will allow us to separate the performance of GOV.UK for cache hits
from cache misses, which will give us some useful insights into how much
the cache speeds things up, and how many requests are actually cached in

CHANGELOG.md

@@ -9,6 +9,7 @@
 
 ## Unreleased
 
+* Report cache status to RUM ([PR #4537](https://github.com/alphagov/govuk_publishing_components/pull/4537))
 * Add type option to component wrapper helper ([PR #4536](https://github.com/alphagov/govuk_publishing_components/pull/4536))
 * **BREAKING** Change devolved nations component type option ([PR #4535](https://github.com/alphagov/govuk_publishing_components/pull/4535))
 * **BREAKING** Add component wrapper helper to intervention component ([PR #4378](https://github.com/alphagov/govuk_publishing_components/pull/4378))

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

@@ -9,4 +9,10 @@
   // is used when a browser loads a page.
   // [1]: https://github.com/alphagov/govuk-rfcs/pull/148
   LUX.addData('http-protocol', navigationPerformance.nextHopProtocol)
+
+  const serverTiming = navigationPerformance.serverTiming
+  if (serverTiming) {
+    const cacheState = serverTiming.find(entry => ['cacheHit', 'cacheMiss'].includes(entry.name))
+    if (cacheState) { LUX.addData('cache', cacheState.name) }
+  }
 }())