Add FID deprecation warn on web vital measure

When any web vital is measured the FID deprecation warning is logged.
This should ensure that browser based scripts that are measuring web
vitals will log a warning for users to see and action on -- move to
using INP.

Author: ankur22

internal/js/modules/k6/browser/common/frame_session.go

@@ -36,6 +36,8 @@ import (
 
 const utilityWorldName = "__k6_browser_utility_world__"
 
+var fidDeprecationWarningOnce sync.Once
+
 // CPUProfile is used in throttleCPU.
 type CPUProfile struct {
 	// rate as a slowdown factor (1 is no throttle, 2 is 2x slowdown, etc).
@@ -345,6 +347,10 @@ func (fs *FrameSession) parseAndEmitWebVitalMetric(object string) error {
 		return fmt.Errorf("json couldn't be parsed: %w", err)
 	}
 
+	fidDeprecationWarningOnce.Do(func() {
+		fs.logger.Warnf("FrameSession:parseAndEmitWebVitalMetric", "FID has been deprecated and superseded by INP -- https://web.dev/blog/fid")
+	})
+
 	metric, ok := fs.k6Metrics.WebVitals[wv.Name]
 	if !ok {
 		return fmt.Errorf("metric not registered %q", wv.Name)