Can I use this hose with my air compressor?
If the metal button that you refer to is this one:



Best guess is that you press this when engaging the end of the gun onto and when disengaging the gun from the valve stem. When the gun is fully pressed onto the valve stem letting go of this button helps clamp the tip of the gun onto the threaded end of the valve stem. 

Again this is an educated guess because I've not seen your gun first hand. You could probably look at the tip of the gun assembly and toggle the metal tab back and forth and see right away what it does. 

Edit

In looking more closely at your picture it appears that the tip of the gun can have various ends slid into it and this lever is designed to hold the tip in place. Pressing the tab (button) releases the catch and allows removal of the tip. 

When I refer to the tips I mean one of these from the kit: