add global callback for refresh latency (#25944)

Co-authored-by: JJ Kasper <[email protected]>

Author: sokra

packages/next/client/dev/error-overlay/hot-dev-client.js

@@ -179,7 +179,11 @@ function onFastRefresh(hasUpdates) {
     DevOverlay.onRefresh()
   }
 
-  console.log('[Fast Refresh] done')
+  const latency = Date.now() - startLatency
+  console.log(`[Fast Refresh] done in ${latency}ms`)
+  if (self.__NEXT_HMR_LATENCY_CB) {
+    self.__NEXT_HMR_LATENCY_CB(latency)
+  }
 }
 
 // There is a newer version of the code available.
@@ -188,11 +192,14 @@ function handleAvailableHash(hash) {
   mostRecentCompilationHash = hash
 }
 
+let startLatency = undefined
+
 // Handle messages from the server.
 function processMessage(e) {
   const obj = JSON.parse(e.data)
   switch (obj.action) {
     case 'building': {
+      startLatency = Date.now()
       console.log('[Fast Refresh] rebuilding')
       break
     }