Please add add_tag_prefix parameter for in_http plugin

[LukasJerabek]

Is your feature request related to a problem? Please describe.

If someone manages my other servers and I tell him that I would like him to send me monitoring data a my rest endpoint created by fluentd, he can obey what I have said a specify in the request, or he wouldnt and use something else that would conflict with my other tags that I use in my fluentd config, potentially.

If you (reader) are not familiar with how in_http works, then let me explain. Whatever you add behind the slash on the endpoint url is used for a tag and it cant be limited right now.

Describe the solution you'd like

I would like to solve this by specifing some tag prefix in source section of fluentds in_http plugin, so I would have his messages marked right away and I could process only those with and dump the rest.

Describe alternatives you've considered

I have considered putting nginx in front of fluentd to prevent any other paths than those with , but prefix would solve this much more effortless.

Additional context

add_tag_prefix already works great in in_forward plugin.