We introduced `prometheus.server_address` setting in `gitlab.yml` which
is to be Rails application's SSOT regarding a Prometheus address.
Hence, we can deprecate the existing settings `enable` and
`listen_address`.

Changes:
* To `Gitlab::Prometheus::Internal` class methods:
  * `server_address` method renamed to `uri_without_protocol`, to
    reflect what the method actually does.
  * `listen_address` method renamed to `server_address`.
  * `promtheus_enabled?` method now depends on presence of
    `server_address`.

* To `Gitlab::DatabaseImporters::SelfMonitoring::Project::CreateService`
  class methods:
  * `prometheus_listen_address` method renamed to
    `prometheus_server_address`.
  * `internal_prometheus_listen_address_uri` renamed to
    `internal_prometheus_server_address_uri`.
Signed-off-by: Balasankar "Balu" C <balasankarc@autistici.org>