Probably ring worm
no
One possibility is a yeast infection.
No - but it can cause you to act like a rash red-neck...
No, this would be very bad. First metronidazole cream is a yeast infection cream - it has an anti-fungal medication in it that will do nothing to clear up an allergic rash and could make the rash worse. Second, metronidazole cream is formulated in a lotion base that it not safe for eating; however, your dog can easily lick the cream off the irritated portion of his belly and could have severe reactions to that medication. Third, it is pretty unusual for a dog to have a specific rash from grass just on his belly - most dogs run around, jump, bounce and roll on the grass when they finallly get out in the spring. It is more likely the rash is a skin infection that needs to be treated with antibiotics from your veterinarian.
It could possibly be pityriasis rosea . A large herald patch usually appears then the rash which is predominantly on the trunk.
Measles
Yes. Even a rash accompanied by itching could seem to be an STD and just be a simple rash.
Among other things, it could be shingles (which might be contagious).
You're not likely to pass cold sores from your face to your neck. Cold sores like to transfer from a wet place like the mouth to another wet place like the genitals or eyes. The rash on your neck could be a sweat rash or a bacterial infection.
it could be a rash if you allergic to something or it could be from a poison plant like poison ivy.
It could be heat rash from sweating.
It would be a "butterfly rash" which could be a sign of lupus. Unless you have pre existing conditions, it's unlikely that it is lupus. It could also be Fifth Disease, which is much more common in children than adults. A fever and rash on the cheeks are followed by flu-like symptoms. It could also be a plain old allergic reaction.