Add some more metrics to the signer (#957)
Add some more metrics to the signer (#957)

* Add some more metrics to the signer

Allow people to know that a log is being signed and there's nothing in
the queue for it. Aim is to help with knowing when a log can be safely
frozen.

* Add a metric for failed signing passes by log id

* Fix typo

daviddrysdale/trillian,gdbelvin/trillian,google/trillian,Martin2112/trillian,RJPercival/trillian,google/trillian,Martin2112/trillian,Martin2112/trillian,AlCutter/trillian,daviddrysdale/trillian,RJPercival/trillian,gdbelvin/trillian,gdbelvin/trillian,daviddrysdale/trillian,AlCutter/trillian