Add v2 and v3 source interface addresses to metadata endpoint

[zenmonkeykstop]

Description

The Source Interface exposes a /metadata endpoint that includes information about an instance's OS and application version, supported languages, and submission key fingerprint, which is useful for Nagios monitoring purposes among other things. Adding the SI addresses in this endpoint would a) allow FPF to monitor v3 service uptake and update Nagios checks accordingly, and b) allow end users on the v2 version of service to verify the correct v3 address for the service (as an alternative or supplement to automatic redirection via the Alt-SVC header).

Potential downside: if an admin turns on v3 but doesn't want to advertise that they've done so, this could inadvertently expose the v3 address.

User Research Evidence

I have none but folks seem to like the idea on Gitter.

User Stories

• As an FPF support team member, I'd like to be able to have v3 service information available for monitoring purposes
• as a SecureDrop user, I'd like to be able to verify the correct v3 address corresponding to a v2 address.