[doc] Add bulk flush configuration options for Kafka output

Author: zhudejun1

docs/reference/filebeat/filebeat-reference-yml.md

@@ -2052,6 +2052,14 @@ output.elasticsearch:
   # is 0.
   #bulk_flush_frequency: 0s
 
+  # The best-effort number of bytes needed to trigger a flush. If the batch reaches 
+  # this size, it will be sent immediately. 0 means disabled. The default is 0.
+  #bulk_flush_bytes: 0
+
+  # The best-effort number of messages needed to trigger a flush. If the batch reaches
+  # this count, it will be sent immediately. 0 means disabled. The default is 0.
+  #bulk_flush_messages: 0
+
   # The number of seconds to wait for responses from the Kafka brokers before
   # timing out. The default is 30s.
   #timeout: 30s

docs/reference/filebeat/kafka-output.md

@@ -276,6 +276,13 @@ The maximum number of events to bulk in a single Kafka request. The default is 2
 
 Duration to wait before sending bulk Kafka request. 0 is no delay. The default is 0.
 
+### `bulk_flush_bytes` [_bulk_flush_bytes]
+
+The best-effort number of bytes needed to trigger a flush. If the batch reaches this size, it will be sent immediately. 0 means disabled. The default is 0.
+
+### `bulk_flush_messages` [bulk_flush_messages]
+
+The best-effort number of messages needed to trigger a flush. If the batch reaches this count, it will be sent immediately. 0 means disabled. The default is 0.
 
 ### `timeout` [_timeout_4]