expose memberlist count in telemetry

Internal users have reported that having each server emit a count of the memberlist (each potentially having its own view of that count) would be a useful production metric.

Start by locating Nomad's memberlist integration and existing telemetry emission points. Determine where each server can report its local memberlist count, then verify that the resulting production metric is emitted consistently; the issue names no files or tests to use.

Written by the indexing model from the issue text.