Support auth basic authentication

[cmizzi]

Many of Prometheus exporters support basic auth authentication. This PR adds this feature to this exporter.

server:
  host: 0.0.0.0
  port: 9144
  auth_basic:
    enabled: true
    username: prometheus
    password: $2y$10$piR5rO00ZYPEEf76Lm7DDeIYRZpSuGR1TuZ4rC7tpxuZmf6w0NU2q // test