BUG?: Internal signals do not run metadata callbacks?

[tangkong]

Current Behavior

The aggregate signals in lightpath_summary sometimes don't believe internal signals it holds onto are connected. We do ask that callbacks be run when we subscribe, but somewhere this isn't happening for InternalSignal

Expected Behavior

Internal signals should report themselves as connected

Context / environment

looking at sp1l0, which had its lightpath_summary not be connected.
Credit to @ZLLentz for debugging this live

Steps to Reproduce (for bugs)

1. Load sp1l0
2. sp1l0.connected = False
3. sp1l0.lightpath_summary.connected = False
4. [sig for sig in sp1l0.lightpath_summary._signals.values() if sig.connected] is empty
5. sp1l0.xtal_out._run_metadata_callbacks()
6. [sig for sig in sp1l0.lightpath_summary._signals.values() if sig.connected] is now not empty

Suggested Solution

Call ._run_metadata_callback at some point, ideally on subscribe (which should be happening...), or possible on init/set/get

[klauer]

We have a hack for this in typhos and proposed a fix in ophyd a few years ago:
pcdshub/typhos#267