[2.11] Move documentation for metrics from the 'metrics' to 'doc' repo

[andreyaksenov]

Previous step #4256

Where the 2.11 metrics docs live:

• https://github.com/tarantool/metrics/tree/master/doc
• https://github.com/tarantool/grafana-dashboard/tree/master/doc

Where to move these docs:

• https://github.com/tarantool/doc/tree/2.11/doc

Then, https://github.com/tarantool/metrics/tree/master/doc and https://github.com/tarantool/grafana-dashboard/tree/master/doc can be removed.

Note

New documentation for metrics will be processed using TarantoolBot.