Compression

[deadtrickster]

• Add compression to metrics endpoint (just gzip is fine) with threshold (say 512 bytes).
• Investigate state of compression in the Elli land. Size metrics from Elli 2 count raw uncompressed values. If we already have 'standard' compressing middleware, I want to have 'http_request_bytes_sent' metric, i.e. metric which counts wire bytes.
• Depending on 2nd task note preferred position for elli_prometheus in a middlewares stack (e.g. it must be first so we properly count raw and network bytes).

[deadtrickster]

I decided to go without threshold because virtually everyone wants Erlang VM metrics and we are always above 512 threshold (which is IIRC used by official clients) then.