[Nickj]

The metal bits are anti rattle shims, you usually find them where you will have some lateral float in caliper or rotor. If you look at these they usually cover the piston area and they are often slightly offset and need to be positioned relative to the rotors rotation.
You want the trailing edge of the pads to hit the disc first or you get a brake that feels really on/off. It will be just a few thou rather than a few mm
Floating caliper or floating disc ... in either case the pads will float over the rotor and maintain clearance
UNLESS the pads or a pad don't float back properly then as soon as one edge touches the disk rotor when the float in the rotor/caliper allows a pad surface to work like an auto hill start device. typically just one pad (If it's both then something is actually broken). Rotate the correct way and it clears itself. If there is some rotation in the caliper mount then it is easier for this to occur
Often there's some friction in the system that isn't helping, often just a layer of scrubbed off pad material stopping the rear face clearing properly (symptom is wedge shaped pads). The old pad material will build up and be quite hard as it gets bloody hot.

Probably a good idea to clean the caplier(s) when using soft pads on a more frequent every so often basis and ensure that any dust is shifted and any heavier 'baked' on stuff is too. The pins need to be round and smooth with no rough corrosion or wear steps, surface damage will stop the pads moving properly and if the pad is left with a little clearance with the piston it can start to rotate as the brakes applied and start to wedge.

I've found disc systems much easier to sort out than stuff like fontana 4LS drums... another story there are you can get a bit of auto servo effect enhancement when there's wear which is fun