Add latency metrics for logstash async output (#42565)

* Add latency metrics for logstash async output

* Properly handle per-batch latency

(cherry picked from commit accc5e1)

Author: strawgate

libbeat/outputs/logstash/async.go

@@ -221,7 +221,8 @@ func (c *asyncClient) sendEvents(ref *msgRef, events []publisher.Event) error {
 		window[i] = &events[i].Content
 	}
 	ref.count.Add(1)
-	return client.Send(ref.callback, window)
+
+	return client.Send(ref.customizedCallback(), window)
 }
 
 func (c *asyncClient) getClient() *v2.AsyncClient {
@@ -231,7 +232,15 @@ func (c *asyncClient) getClient() *v2.AsyncClient {
 	return client
 }
 
-func (r *msgRef) callback(n uint32, err error) {
+func (r *msgRef) customizedCallback() func(uint32, error) {
+	start := time.Now()
+
+	return func(n uint32, err error) {
+		r.callback(start, n, err)
+	}
+}
+
+func (r *msgRef) callback(start time.Time, n uint32, err error) {
 	r.client.observer.AckedEvents(int(n))
 	r.slice = r.slice[n:]
 	r.deadlockListener.ack(int(n))
@@ -246,6 +255,11 @@ func (r *msgRef) callback(n uint32, err error) {
 			r.win.tryGrowWindow(r.batchSize)
 		}
 	}
+
+	// Report the latency for the batch of events
+	duration := time.Since(start)
+	r.client.observer.ReportLatency(duration)
+
 	r.dec()
 }