Add Logstash-compatible formatter

[springerigor]

ELK Stack (Elasticsearch, Logstash & Kibana trio) is commonly used today for collecting and presenting logs.

Logstash, to parse logs correctly, needs to have them delivered in a compatible JSON format:

{
  "message"    => "hello world",
  "@version"   => "1",
  "@timestamp" => "2018-10-09T23:03:14.111Z",
  "type"       => "stdin",
  "host"       => "hello.local"
}

Only @timestamp & @version are required.

• @timestamp is the ISO8601 high-precision timestamp for the event.
• @version is the version number of this json schema

Every other field is valid and fine.

To support Logstash format by default I introduced a brand new LogstashFormatter. It generates a compatible JSON message with the following fields:

{
  "@timestamp" => "2018-10-09 17:54:58 +0200",
  "@version" => 1,
  "content_length" => "512",
  "http_method" => "GET",
  "message" => "[HTTParty] 200 \"GET http://my.domain.com/my_path\" - ",
  "path" => "http://my.domain.com/my_path",
  "response_code" => 200,
  "severity" => "info",
  "tags" => ["HTTParty"]
}

The required fields are present + some additional ones which I found useful to have in Kibana for filtering & debugging purpose. Looking forward to your comments.