switch prometheus target discovery from file_sd and exporter resources to puppetdb

we currently use Puppet exported resources to configure the Prometheus server to scrape our various targets. this works somewhat okay, but requires a lot of exported resources and takes a while to propagate, and is error prone.

there's a much easier way which doesn't require exported resources at all, the puppetdb_sd_config parameter, which basically pulls a list of targets from PuppetDB dynamically.

That code was added in Prometheus 2.31, so this won't work until Debian bookworm is released (or backports), but it is nevertheless a very promising tool.

See also this sample configuration for more ideas on what this can do.