DNSSEC sign metrics: CHANGES, doc, release note

Add some information about the new statistic-channel DNS sign
metrics. Also add a CHANGES and release note entry.

(cherry picked from commit 3a3f40e3)
39 jobs for !2080 with 513-matthijs-dnssec-sign-stats-v9_14 in 12 minutes and 43 seconds (queued for 3 seconds)
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