Introduce logging JsonLayout.

[azasypkin]

In this PR we introduce new JsonLayout that can be used by logging appenders to output JSON strings instead of human-readable log messages.

Fixes #12941

Depends and based on #12852

Can be tested with the following config example:

logging.appenders.default:
  kind: console
  layout.kind: json

[azasypkin]

Output looks like this:

{"timestamp":"2017-07-31T10:39:40.411Z","level":{"id":"info","value":5},"context":"elasticsearch.client.admin","message":"cluster client stopped"}

But I don't know use case for JSON output, really (cc @epixa, @tylersmalley). Maybe we need something like this instead:

format({ timestamp, level, context, message, error, meta }: LogRecord): string {
    // Per spec `timestamp.toJSON()` uses `timestamp.toISOString()` under the hood.
    // See http://www.ecma-international.org/ecma-262/5.1/#sec-15.9.5.44
    return JSON.stringify({
      '@timestamp': timestamp,
      level: level.id,
      context,
      message,
      error: error && error.message,
      meta
    });
  }

{"@timestamp":"2017-07-31T10:35:49.230Z","level":"info","context":"elasticsearch.client.admin","message":"cluster client stopped"}

[kimjoar]

++ I prefer the "more specific" format. We should also take a look at current Kibana to make sure we're "fairly consistent", but I'm good with doing that in a follow-up if you want to

[azasypkin]

Yeah, will make it closer to what we have now in Kibana (it will be trivial to tweak it later anyway):

{  
   "type":"log",
   "@timestamp":"2017-07-31T12:24:32Z",
   "tags":[  
      "warning",
      "elasticsearch",
      "admin"
   ],
   "pid":32107,
   "message":"Unable to revive connection: http://localhost:9200/"
}

and here is what we have in Elasticsearch:

{
  "timeMillis" : 1501510600593,
  "thread" : "Thread-2",
  "level" : "INFO",
  "loggerName" : "org.elasticsearch.node.Node",
  "marker" : {
    "name" : "[SRbYZv9] "
  },
  "message" : "closed",
  "endOfBatch" : false,
  "loggerFqcn" : "org.apache.logging.log4j.spi.AbstractLogger",
  "threadId" : 15,
  "threadPriority" : 5
}

[azasypkin]

Filled dedicated issue to discuss what additional data we should log, especially relevant for JSONLayout: #13241

[kimjoar]

LGTM

[kimjoar]

Obviously -> The?

[kimjoar]

++ I prefer the "more specific" format. We should also take a look at current Kibana to make sure we're "fairly consistent", but I'm good with doing that in a follow-up if you want to

[azasypkin]

Merging now, we'll be able to tune format later in the scope of #13241.