cephadm/monitoring: Update cephadm web endpoint to provide scrape configuration information to Prometheus
Starting with prometheus v2.28, prometheus.yml can fetch the scrape configurations from a http endpoint.
Documentation of the format required can be found here https://prometheus.io/docs/prometheus/2.28/configuration/configuration/#http_sd_config
By offering this feature, we simplify the internal configuration of the prometheus that cephadm deploys whilst also providing support for users to monitor ceph using an external Prometheus environment - ensuring their scrape configurations are always in line with the cluster configuration.
Obviously to make full use of this feature cephadm will need to deploy Prometheus 2.28 or newer (current version is 2.33 Feb 2022)