I'll bite here. I'll agree that the first part of the post is easy, using a
shunt and a meter across the shunt, but the second part of the post requests
that it measure the current while being powered from the same supply.

I can easily set this up with a battery powered meter, to measure the
current in the supply, but to power the meter from the same supply stumps
me......

Assuming this is for a meter to monitor the aircraft's power use after say
the master switch, then the only way I see to have the unit self powered is
to use a normal analogue moving needle gauge where it is self powered.

Peter