Add docs for http output to send tag in header

output/http.md

@@ -15,6 +15,7 @@ The __http__ output plugin, allows to flush your records into an HTTP end point.
 | Proxy | Specify an HTTP Proxy. The expected format of this value is _http:https://host:port_. Note that _https_ is __not__ supported yet. ||
 | URI | Specify an optional HTTP URI for the target web server, e.g: /something | / |
 | Format | Specify the data format to be used in the HTTP request body, by default it uses _msgpack_. Other supported formats are _json_ and _json_stream_. | msgpack |
+| header_tag | Specify an optional HTTP header field for the original message tag. | |
 
 ### TLS / SSL
 
@@ -54,3 +55,39 @@ In your main configuration file append the following _Input_ & _Output_ sections
  Port 80
  URI /something
 ```
+
+By default, the URI becomes tag of the message, the original tag is ignore. To
+retain the tag, multiple configuration sections has to be made based and flush
+to different URIs.
+
+Another approach we also support is the sending the original message tag
+in a configurabled header. It's up to the receiver to do what it want with that
+header field: parse it and use it as the tag for example. With fluend
+http plugin, it's straightforward.
+
+To configure this behaviour, add this config:
+
+```
+[OUTPUT]
+ Name http
+ Match *
+ Host 192.168.2.3
+ Port 80
+ URI /something
+ header_tag FLUENT-TAG
+```
+
+Given the default http input fluentd plugin: https://github.com/fluent/fluentd/blob/1afbfb17c833b05757122a53ea14b17af659fd75/lib/fluent/plugin/in_http.rb#L212-L215
+
+We can easily parse the tag like this:
+
+```ruby
+@@ -153,6 +153,7 @@ module Fluent::Plugin
+ begin
+ path = path_info[1..-1] # remove /
+ tag = path.split('/').join('.')
++ tag = params["HTTP_FLUENT_TAG"] if params["HTTP_FLUENT_TAG"]
+ record_time, record = parse_params(params)
+```
+
+Notice how we override the tag, which is from URI path, with our custom header