API key authentication is now possible by making use of the X-API-Key
header. API authentication will only be enabled when a user adds their
first API key, up until this point, they can still use their username
and password to authenticate with API endpoints. After the user adds
their first API key, they may only use API keys in future to
authenticate with the API.
In this commit, the prometheus /metrics endpoint has been changed over
to the new authentication system.
Signed-off-by: Matthew Nickson <mnickson@sidingsmedia.com>