fix: add sentAt when put item into pendingQueue (#509)

Signed-off-by: jonyhy96 <hy352144278@gmail.com>

Signed-off-by: jonyhy96 hy352144278@gmail.com

Fixes #508
diff --git a/pulsar/producer_partition.go b/pulsar/producer_partition.go
index 87c97ed..b365f83 100644
--- a/pulsar/producer_partition.go
+++ b/pulsar/producer_partition.go
@@ -547,6 +547,7 @@
 			continue
 		}
 		p.pendingQueue.Put(&pendingItem{
+			sentAt:       time.Now(),
 			batchData:    batchesData[i],
 			sequenceID:   sequenceIDs[i],
 			sendRequests: callbacks[i],