feat: add metric missing in kafka v4 (#376)

This mbean `bufferpool-wait-time-total` was renamed in kafka v4 to
`bufferpool-wait-time-ns-total`

Author: abhishuraina

src/metrics/producer_definitions.go

@@ -54,6 +54,11 @@ var producerMetricDefs = []*JMXMetricSet{
 				SourceType: metric.GAUGE,
 				JMXAttr:    "client-id=" + producerHolder + ",attr=bufferpool-wait-time-total",
 			},
+			{
+				Name:       "producer.bufferpoolWaitTime",
+				SourceType: metric.GAUGE,
+				JMXAttr:    "client-id=" + producerHolder + ",attr=bufferpool-wait-time-ns-total",
+			},
 			{
 				Name:       "producer.bytesOutPerSecond",
 				SourceType: metric.GAUGE,