Integrate MON and Prometheus

Using the metrics-endpoint | prometheus_scrape  integration
Add prometheus-url to the charm config.
prometheus-url does not have a default value.
if prometheus-url is missing the charm is blocked.

Change-Id: I963f305efec9f171eace82379ba23999736f1d4e
Signed-off-by: Patricia Reinoso <patricia.reinoso@canonical.com>