I have never been to Puerto del Carmel. However, I have been to Spain many times. I do not hide when I inject so, although I do not inject "publicly", I usually inject "in public".
At no time have I experienced any problem with anyone in a restaurant in Spain (or anywhere else come to that). So, if I was you, I'd just go, have fun and manage your diabetes in the same way as you do at home. Just remember to keep your insulin cool.
Have a great time.