[TCM] Metrics reference

**Product:** Tarantool Cluster Manager
**Since:** TCM 1.0.0, Tarantool 3.0
**Audience/target:** admins
**Root document:** new page in the TCM section
**SME:** @ filonenko-mikhail 

# Details

Write the TCM metrics reference.
All descriptions are available in `<tcm_uri>/metrics`.

Consider writing monitoring how-to (check TDG2 docs for reference).