Retrieve one newsletter metric bucketed over time

One metric, bucketed, for charting. Every other operation in this domain answers with a snapshot, which is the wrong shape for a line, so this one returns the series behind it.

Name the quantity with metric and the bucket width with interval. Buckets are contiguous, and a bucket in which nothing happened comes back with a value of 0 rather than being omitted, so a client can plot the array without filling the gaps itself.

subscribers counts subscriptions Commune recorded in each bucket, not the running audience size, and it inherits the partial cache caveat that applies wherever Commune counts the subscribers of a newsletter connected to an outside provider. sends counts the dispatches Commune performed and is never a count of every email the newsletter has sent, because an imported article was mailed by the provider before Commune saw it.

Needs insights: read. threads and messages depend on whether the key acts for the newsletter, which a key does when it holds write in any family. Such a key counts every conversation. A key that holds only read permissions counts public conversations and the replies in them, which leaves out an article's own discussion, visible to subscribers only. The other four metrics are the same for both kinds of key.