inclusion-checker: add Prometheus gauge for unseen certs

[cpu]

I found this stat to be useful while I was working on the ct-test-srv and gives a picture of the backlog of unseen certs that are outstanding beyond just knowing the age of the oldest in the backlog.

[coveralls]

Pull Request Test Coverage Report for Build 263

• 1 of 1 (100.0%) changed or added relevant line in 1 file are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage increased (+0.03%) to 66.495%

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Totals
Change from base Build 257:	0.03%
Covered Lines:	645
Relevant Lines:	970

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💛 - Coveralls