ceph-exporter: add support for launching the exporter daemon

[piyushagarwal1411]

This PR adds support for optionally launching the ceph-exporter daemon in the Ceph environment setup. A new --exporter command-line option is introduced to the vstart.sh script (ceph/ceph#60623), allowing users to enable or disable the exporter daemon. The changes include:

1. docker-compose.yml:

   • Added a new environment variable EXPORTER with a default value of 0. The variable can be set to 1 to enable the exporter.

2. docker/ceph/rpm/Dockerfile:

   • Included the installation of ceph-exporter in the Docker image by adding it to the dnf install command, ensuring that the exporter daemon is available in the container.

3. docker/ceph/set-start-env.sh:

   • Modified the script to check for the EXPORTER flag. If set to 1, the --exporter option is added to the VSTART_OPTIONS, triggering the startup of the exporter daemon when the environment is initialized.

4. docker/prometheus/ceph-exporter-targets.yml (new file):

   • Added a new configuration file to define the ceph-exporter targets for Prometheus scraping.

5. docker/prometheus/prometheus.yml:

   • Configured Prometheus to scrape metrics from the ceph-exporter by adding a new job for the exporter and using the ceph-exporter-targets.yml file for the target configuration.