Support HTTP stream as used by journald-upload

[ThorbenJ]

Opening this here, as a follow up to: elastic/logstash#2594

Journald has a native way to send logs to a remote system, and it would be nice if Logstash could receive them.

Journald uses HTTP streaming. i.e. Open connection with some headers and the first message, the server then gives "100 Continue" and the next message is sent, ad-infinitum:

https://www.freedesktop.org/software/systemd/man/systemd-journal-upload.html

This sort of streaming does not yet seem easily possible with the HTTP input plugin.

[johntdyer]

yea, we need this

[JonnyHaystack]

Has there been any change here in the last 2 years? Would be very nice to have systemd-journal-upload transfer directly to logstash.