Allow configuration of Prometheus storage.tsdb.retention.time

[geerlingguy]

I want to increase the retention time for Prometheus data so I can better compare long-term data. Right now it just defaults to 15d (15 days).

For now, I'm considering just setting --storage.tsdb.retention.time=90d in the docker-compose.yml file and keeping it at that for the time being.

For easier configurability, for now we'd have to template the docker-compose.yml file and change that value when the playbook templates it. See: https://github.com/prometheus/prometheus/issues/6188 and https://github.com/prometheus/prometheus/issues/9115