Performance: Use median to stabilize the First Block metric (#54157)

* Calculate median instead of average for the FBL metric
* Apply median to all loading metrics for stability

test/performance/config/performance-reporter.ts

@@ -81,12 +81,12 @@ export function curateResults(
 		timeToFirstByte: median( results.timeToFirstByte ),
 		largestContentfulPaint: median( results.largestContentfulPaint ),
 		lcpMinusTtfb: median( results.lcpMinusTtfb ),
-		serverResponse: average( results.serverResponse ),
-		firstPaint: average( results.firstPaint ),
-		domContentLoaded: average( results.domContentLoaded ),
-		loaded: average( results.loaded ),
-		firstContentfulPaint: average( results.firstContentfulPaint ),
-		firstBlock: average( results.firstBlock ),
+		serverResponse: median( results.serverResponse ),
+		firstPaint: median( results.firstPaint ),
+		domContentLoaded: median( results.domContentLoaded ),
+		loaded: median( results.loaded ),
+		firstContentfulPaint: median( results.firstContentfulPaint ),
+		firstBlock: median( results.firstBlock ),
 		type: average( results.type ),
 		minType: minimum( results.type ),
 		maxType: maximum( results.type ),