Add metric-names subcommand.

[fujiwara]

Hi. I added metric-names subcommand.

metric-names fetches metric names of the host or service.

mkr metric-names --help
NAME:
   mkr metric-names - Fetch metric names

USAGE:
   mkr metric-names [command options] [--host | -H <hostId>] [--service | -s <service>] [--jq <formula>]

DESCRIPTION:
   
    Fetch metric names of 'host metric' or 'service metric'.
    Requests "/api/v0/hosts/<hostId>/metric-names" or "/api/v0/services/<serviceName>/metric-names".
    See https://mackerel.io/ja/api-docs/entry/hosts#metric-names, https://mackerel.io/ja/api-docs/entry/services#metric-names


OPTIONS:
   --host value, -H value     Fetch host metric names of <hostID>.
   --service value, -s value  Fetch service metric names of <service>.
   --jq value                 Filter response values using jq syntax

[lufia]

Hi! Thank you for your contributions.

We are going to review this PR next week.

[yseto]

seems good. please rewrite commands Description.