* client/finality-grandpa: Instrument until-imported queue
The `UntilImported` queue takes as input finality grandpa messages that
depend on blocks that are not yet imported and holds them back until
those blocks are imported.
This commit adds a basic metric, the amount of messages waiting in the
queue, to the module. For now this metric is only available for the
global `UntilImported` queue awaiting blocks for commit and catch-up
messages.
* client/finality-grandpa/src/until_imported: Update metric help text
Co-Authored-By: Ashley <ashley.ruglys@gmail.com>
Co-authored-by: Ashley <ashley.ruglys@gmail.com>