When an equal collector was already registered, the caller kept using
its own instance, whose observations are never scraped. RegisterCollector
now returns the registered collector so callers can adopt it.
Registration errors were discarded, leaving the collector silently
absent from the metrics endpoint while the vectors still recorded.
Duplicate registrations remain ignored; they are expected when several
instances are constructed in the same process.
Move metrics and probe listeners from main.go into server.go and
manage all servers with errgroup. This replaces log.Fatalf goroutines
with proper error propagation and ties all server lifetimes together
so a failure in any listener triggers graceful shutdown of the others.
Trong Huu Nguyen
Morten Lied Johansen
Kim Tore Jensen