hs: New metrics module

At this commit, a new service registers to the module and a store is created.
It also remove itself from the metrics module if it goes away.

In order to hook into the metrics subsystem, this commit attaches the HS
subsystem into the subsystem global list so its get_metrics() call can be
accessible.

HS initialization is still _not_ done through the subsys module as it is
likely require much more testing.

Related to #40063

Signed-off-by: David Goulet <dgoulet@torproject.org>