The RCON protocol won't get you where you want to be, really. It allows you to query for the current status (ie, the number of people connected, map, score, teams, etc) but for any real statistics you're going to want to check the log file.
There are console/config commands that allow the log to be transmitted over UDP as well as written to disk, so knowledge of RCON can be important depending on what your goals are.
The log file standard is documented on the Valve developer Wiki. You can write an offline or realtime parser for the log, which can allow you to collect all sorts of interesting statistics.
There are also "SuperLog" plugins for most Source engine games which will augment the default log options and give you more info than you'd normally have. See this thread for more info.
As you may imagine, this is actually a popular thing, so several stats programs exist already. The one I'm most familiar with is HLX:CE since I've contributed to the source in the past. From what I recall, setting it up requires some technical know-how, but it's free.
If you don't mind paying $5/month, you might check out gameME which is a popular paid alternative. The money you invest in it is offset by the more "hands off" approach to the software.