What exactly does 'Purge Resources' do? Will that clear/wipe out the historical metric data from the database?
The purge function does not delete metrics from the database; it only clears the list of resource names. When a resource list is cleared it will then begin to repopulate with resources monitored in the traffic from the time of the purge onward. Resources that reappear in the list will have their entire set of historical data intact. Metrics for "obsolete" resources that no longer appear in the network traffic will not be accessible after the purge. All metrics will be preserved, but if a resource is not accessed after the purge it will never appear in the user interface again.
In other words, purging just drops the names, not the data. The name list will repopulate as new metrics are added to the system. The purges can take awhile--up to 30 minutes. The way you will know a purge is complete is, in the web console UI, to click on ('Analysis' | 'Resource List'), choosing the category you purged, and getting an empty list.
(Note: When configuring new systems, you might have been initially monitoring more servers, ports, and/or sites you wanted to be monitoring. And so you made configuration changes so that Foglight Experience Monitor (FxM) monitors a smaller subset of all the web traffic. So it often happens that the database gets populated with "obsolete" names for URLs that will no longer appear again in the metrics. The system will eventually "drop" these from the database but that process could take weeks and bloats the size of the database in the meantime. So, doing a purge of Pages and Page Elements is standard operating procedure for all of our installations after the configuration phase is complete. It will make the system more efficient and improve performance.)
With regards to how the appliance retain data in the database (retention policy), please refer to section "Setting Database Retention" in the Foglight Experience Monitor : Installation and Administration Guide.