Trace cold request waterfalls and isolate the layout discovery regression

perf/proxy.mjs

@@ -47,6 +47,7 @@ export function categorizeContentType(contentType, urlPath) {
  */
 export function createThrottleProxy({ targetPort, profile = 'lte' }) {
   let activeTracking = false;
+  let trackingStart = 0;
   let requestCount = 0;
   let requests = [];
   let totalWireBytes = 0;
@@ -79,6 +80,7 @@ export function createThrottleProxy({ targetPort, profile = 'lte' }) {
       return;
     }
 
+    const requestStart = Date.now();
     reqIndex++;
     const currentReqIndex = reqIndex;
 
@@ -114,7 +116,8 @@ export function createThrottleProxy({ targetPort, profile = 'lte' }) {
         let recorded;
         if (activeTracking) {
           requestCount++;
-          recorded={url:clientReq.url,status:upRes.statusCode,bytes:0};
+          recorded={url:clientReq.url,status:upRes.statusCode,bytes:0,index:currentReqIndex,startMs:requestStart-trackingStart,headersMs:Date.now()-trackingStart};
+          clientRes.once('finish',()=>{recorded.endMs=Date.now()-trackingStart;});
           requests.push(recorded);
         }
 
@@ -176,6 +179,7 @@ export function createThrottleProxy({ targetPort, profile = 'lte' }) {
         server.close(resolve);
       }),
     startTracking: () => {
+      trackingStart = Date.now();
       activeTracking = true;
     },
     stopTracking: () => {