This commit adds in an http listener that provides a `/metrics`
endpoint that can be configured as a target by prometheus to
scrape and record metrics.
You may also curl it for yourself.
While playing around with this, I found and fixed a deadlock
in the case that the initial spool in logic found and expired
a message; it would reuse a helper function that wanted to acquire
a lock, but it already held the lock.
* Switch uuids to v1 format, so that we can cheaply determine
when a message was created without having to load its metadata
from the spool
* Add some message delivery parameters; retry interval, limit, max age
* Respect those parameters when spooling in and when we encounter
a transient failure.
A callback is used to resolve the config at the time that
we set up the destination site structure (eg: when we get the first
ready-to-send message for it, and again after it has idled out).
This is used in the example configs to enable "OpportunisticInsecure"
TLS mode so that we can successfully delivery over TLS to the
default self-signed TLS certs on the sink.
Wez Furlong