When I am setting up a tor relay, one option is to determine the contact information as an email address. Is it secure considering information harvesting by NSA for instance or would be better to hide it?
Email information is publicly available so you'd better use a special email address for your relay. Either set up a fresh one (optionally via Tor) or check if your email provider gives the opportunity to set up email aliases. This way you can handle better spam bots.
Email contact is useful in case someone wants to reach you and in general is the kindest way to run a relay.
Setting up no email contact will not keep you safe from a surveillance service, as the one you stated. There will always exist other legal or not, sophisticated or not ways for such a state's service to find out the relay operator. Examples : asking (legally or not) your hosting provider, monitoring (legally or not) incoming ssh connections to you relay etc.
All the information about relays are public and everyone can see it on Atlas (Like this one). It's good to have contact information there (specially if you're running an exit relay), in case something goes wrong and someone needs to contact you, but of course it's not required.
I'd suggest you set up a new account over Tor, if you've concerns.
The email address is optional but good to have there if someone needs to contact you about the relay. The address will be harvested and receive spam so you might not want to use your primary address.