Deprecate Praefect's `prometheus_exclude_database_from_default_metrics` configuration

Scraping Praefect DB metrics requires expensive queries. Currently the prometheus_exclude_database_from_default_metrics configuration determines whether the DB metrics get exported to separate endpoint. By default DB metrics are exported to separate /db_metrics endpoint for scraping. This reduces resource consumption by enabling separate configuration of the metrics scraping intervals. It is now desired to deprecate this configuration option in v16.0 in favor of always exporting Praefect DB metrics to a separate endpoint for scraping.