Cherry-pick #19975 to 7.x: Fix starttime of metricsets on dataset endpoint for metricbeat (#20147)

(cherry picked from commit 88cda01)

Co-authored-by: Vijay Samuel <[email protected]>

Author: Steffen Siering

metricbeat/mb/module/wrapper.go

@@ -142,7 +142,7 @@ func (mw *Wrapper) Start(done <-chan struct{}) <-chan beat.Event {
 			defer msw.close()
 
 			registry.Add(metricsPath, msw.Metrics(), monitoring.Full)
-			monitoring.NewString(msw.Metrics(), "starttime").Set(common.Time{}.String())
+			monitoring.NewString(msw.Metrics(), "starttime").Set(common.Time(time.Now()).String())
 
 			msw.run(done, out)
 		}(msw)