lua: embed metrics module

tarantool/metrics [1] is a lua module (distributed as a separate rock)
for metrics aggregation and export. After this patch, it will be a part
of the tarantool binary.

1. https://github.com/tarantool/metrics

Part of #7725

@TarantoolBot document
Title: embedded metrics

Now tarantool has metrics module on its board. metrics is a lua module
previously distributed as a separate rock which is widely used by
tarantool applications. Metrics has its own documentation section:
https://www.tarantool.io/en/doc/latest/book/monitoring/ , but it doesn't
yet mention anything about the embedding.