Set up TarantoolBot for the 'metrics' repository

[andreyaksenov]

Given that starting with v2.11.1 the metrics module is included in Tarantool, docs for metrics are going to live in the main documentation repo: tarantool/doc#4348. This requires a new process of documenting metrics features. A developer needs to mention @TarantoolBot in the commit message and add a description on what's done.

Example commit from the tt repo: https://github.com/tarantool/tt-ee/pull/130/commits/5fd5b54e202a7e277b6dd09ef54727a7f1b437cf