adapt metrics-lib official name to Tor Metrics Library

This only affects documentation. The package renaming is done in task #19616 (moved).

Remove all usages of 'DescripTor' and replace them by 'Tor Metrics Library' or 'Metrics Library'. The technical short name is 'metrics-lib' (i.e., trac)

(optimistically set milestone to 2.0.0)