in_unix: Add tag parameter

Signed-off-by: Masahiro Nakagawa <repeatedly@gmail.com>

Author: repeatedly

plugins/input/unix.md

@@ -44,6 +44,14 @@ Fluentd will use the environment variable `FLUENT_SOCKET` if defined.
 
 The backlog of Unix Domain Socket.
 
+### tag
+
+| type   | default | version |
+|:-------|:--------|:--------|
+| string | nil     | 1.11.0  |
+
+`in_unix` uses incoming event's tag by default. If set `tag` parameter, use its value instead.
+
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