I regularly inject insulin in all sorts of public situations eg yesterday on a bench at the shopping centre, usually no one bats an eyelid. My philosophy is that healthy people secrete insulin whenever they eat naturally, I have to inject but it's basically the same thing, why should I therefore feel embarrassed or ashamed. Woe betide the first member of joe public who makes any sort of non positive comment for they will get the sharp end of my tongue!
Cabin crew should be at least a little bit aware of diabetes as they need to understand some common health conditions that may result in a first aid situation as part of their training.
I once had a bad hypo on a flight, I had my seatbelt on so couldn't reach my supplies, the cabin crew were great and got me some coke to drink straight away.