Oracle provides a number of performance monitoring capabilities; I'm unsure of why you wish to write your own tool. You don't mention the version of Oracle that you're using, but you can start with looking into Statspack, and, if you're using Oracle10g, the various monitoring tools (ADDM, ASH) that are built into the RDBMS.
If you still want to write your own tool, I'd suggest observing the metrics that the Oracle tools use to give you ideas on what to capture. As for the language, that depends on what you want the tool to look like -- you can certainly produce something useful using only SQL and PL/SQL.
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