Adds a `mxaccess-sessions` health check reporting how many MXAccess sessions are
healthy. Each session is one worker process holding one MXAccess COM instance —
a live connection into a Galaxy — so this answers "how many Galaxy connections
are healthy" in the vocabulary the code actually uses.
Zero sessions is Healthy, deliberately, and the rest of the design follows from
that. The gateway opens a session when a client asks and holds none otherwise,
so an idle gateway is working normally. A count threshold ("unhealthy below N")
would sit red forever on a host nothing dials yet, and a permanently red probe
is one operators stop reading — which leaves them worse off than no probe. The
check therefore grades on whether the sessions that exist are usable: nothing
faulted is Healthy, some faulted beside a ready or starting one is Degraded, and
every session faulted is Unhealthy. Counts ride along as entry data for the
family Overview dashboard.
Tagged `active` rather than `ready` for the same reason. Readiness decides
whether the process should be sent traffic, and a gateway with no sessions is
ready to serve — unlike the auth store, which every call depends on. Failing
readiness here would pull a working gateway out of rotation over a condition its
own clients create.
Reads ISessionRegistry, which already exposes Snapshot(); ISessionManager stays
the command surface and grows no enumerator.
Joseph Doherty