stats: the writers of the document

How many writers (Automerge actors) the document's changes have matters: a writer is a
node's run (each start, rejoin or drop makes one, a start keeping the one of before with
`--keep-actor` after a clean stop), and every transfer of the whole history carries a
signature for each. `/admin/stats` now says it (`document.writers`, and
`document_writers` for Prometheus), counted from the changes' metadata alongside the
saved size's measurement in the background (at least a minute apart), as that takes
time proportional to the document too; an export, which measures the size alone, keeps
the count of before. The dashboard's figures show it with the rest.

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