This sounds like one of those "what are you trying to achieve" questions, where you need to step back and ask why you're trying to collect that info and what you want it for.
First, how do you define "application"? Any client that connects to a given database? Any client that sets the application_name GUC to a particular value? Any client from a given host? etc. "Application" can mean a lot of different things. It sounds like you might simply mean work done on a connection that isn't internal to the database system's bookkeeping, which is a bit easier, but I'm not sure.
Second, why? What does the transaction count tell you? How will you account for work done by PgAgent (if used), via dblink, etc?
I guess I'm unsure what you're trying to accomplish.
Thanks Craig for your reply.
4 What's, 2 Why's, 2 How's ... :)
Let me put my question simple.
What counts include in xact_commit & xact_rollback of pg_stat_database ?