The description of the feat Sentinel says:
When a creature within 5 feet of you makes an attack against a target other than you (and that target doesn't have this feat), you can use your reaction to make a melee weapon attack against the attacking creature.
Now I'm wondering, could I potentially use this with Booming Blade since the spell implies the use of a melee weapon attack? I'm unsure if this would count as casting a spell or making a melee weapon attack.