Add prometheus metrics for block authorship (#10316)

* Add prom metric  to basic authorship

* Add proposer_block_proposal_time

* +nightly-2021-10-29 fmt

* Use saturating_duration_since, not elasped

* Update client/basic-authorship/src/basic_authorship.rs

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* Update client/basic-authorship/src/basic_authorship.rs

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>

* +nightly-2021-10-29 fmt

Co-authored-by: Bastian Köcher <bkchr@users.noreply.github.com>