Add support for JSON in metrics commands

[romainneutron]

This address CLI-72
It adds supports for JSON output to metrics commands

[pjcdawkins]

This PR is not limited to the metrics commands but instead adds a json format to all commands that output tables. The code's fine but it raises some questions:

• Would you expect "tabular" JSON data for all the other commands, such as environments --format json, or would you expect the output to match the OpenAPI documentation for the /environments endpoint?
• Does JSON output make sense for all those other commands? Why not call the API directly?
• I think the same question applies to the metrics commands. Could some uses be outlined?
• Does JSON make sense in a Table class?

here's the current output for environments --format json, assuming one environment:

{
  "data": [
    {
      "ID": "main",
      "Title": "main",
      "Status": "Active",
      "Type": "production"
    }
  ]
}

the current way to get JSON output, project:curl /environments, returns a lot more information and it matches the OpenAPI docs