initial prometheus metrics (#1536)

* service-new: cosmetic changes

* overseer: draft of prometheus metrics

* metrics: update active_leaves metrics

* metrics: extract into functions

* metrics: resolve XXX

* metrics: it's ugly, but it works

* Bump Substrate

* metrics: move a bunch of code around

* Bumb substrate again

* metrics: fix a warning

* fix a warning in runtime

* metrics: statements signed

* metrics: statements impl RegisterMetrics

* metrics: refactor Metrics trait

* metrics: add Metrics assoc type to JobTrait

* metrics: move Metrics trait to util

* metrics: fix overseer

* metrics: fix backing

* metrics: fix candidate validation

* metrics: derive Default

* metrics: docs

* metrics: add stubs for other subsystems

* metrics: add more stubs and fix compilation

* metrics: fix doctest

* metrics: move to subsystem

* metrics: fix candidate validation

* metrics: bitfield signing

* metrics: av store

* metrics: chain API

* metrics: runtime API

* metrics: stub for avad

* metrics: candidates seconded

* metrics: ok I gave up

* metrics: provisioner

* metrics: remove a clone by requiring Metrics: Sync

* metrics: YAGNI

* metrics: remove another TODO

* metrics: for later

* metrics: add parachain_ prefix

* metrics: s/signed_statement/signed_statements

* utils: add a comment for job metrics

* metrics: address review comments

* metrics: oops

* metrics: make sure to save files before commit 



* use _total suffix for requests metrics

Co-authored-by: Max Inden <mail@max-inden.de>

* metrics: add tests for overseer

* update Cargo.lock

* overseer: add a test for CollationGeneration

* collation-generation: impl metrics

* collation-generation: use kebab-case for name

* collation-generation: add a constructor

Co-authored-by: Gav Wood <gavin@parity.io>
Co-authored-by: Ashley Ruglys <ashley.ruglys@gmail.com>
Co-authored-by: Max Inden <mail@max-inden.de>