Allow for exporter scraping without username/password

[mjnagel]

Currently I have NeuVector configured to disable local authentication and only allow SSO (see related issue). I'm unable to use the exporter currently because it assumes a user/pass authentication to retrieve a token.

Ideally I would either like to have the metrics available unauthenticated (and use other means to protect them, such as Istio) or provide some mechanism to handle a token exchange with my SSO provider.