out_es: custom http headers

pipeline/outputs/elasticsearch.md

@@ -13,6 +13,7 @@ The **es** output plugin, allows to ingest your records into an [Elasticsearch](
 | Host | IP address or hostname of the target Elasticsearch instance | 127.0.0.1 |
 | Port | TCP port of the target Elasticsearch instance | 9200 |
 | Path | Elasticsearch accepts new data on HTTP query path "/\_bulk". But it is also possible to serve Elasticsearch behind a reverse proxy on a subpath. This option defines such path on the fluent-bit side. It simply adds a path prefix in the indexing HTTP POST URI. | Empty string |
+| header | Add additional arbitrary HTTP header key/value pair. Multiple headers can be set. |  |
 | compress | Set payload compression mechanism. Option available is 'gzip' | |
 | Buffer\_Size | Specify the buffer size used to read the response from the Elasticsearch HTTP service. This option is useful for debugging purposes where is required to read full responses, note that response size grows depending of the number of records inserted. To set an _unlimited_ amount of memory set this value to **False**, otherwise the value must be according to the [Unit Size](../../administration/configuring-fluent-bit/unit-sizes.md) specification. | 512KB |
 | Pipeline | Newer versions of Elasticsearch allows to setup filters called pipelines. This option allows to define which pipeline the database should use. For performance reasons is strongly suggested to do parsing and filtering on Fluent Bit side, avoid pipelines. |  |