Metro is dual licensed -- it's also CDDL. If Metro were purely GPL2, you'd be obligated to release your source code to your customers if they request it.
That means, that if the GPL2 were in effect, your obligation to release the code is only to people to whom you have released implementations. You're under no obligation to release your code to "the public", host some kind of server, or any of that. Your only obligation under the GPL is to your clients, and even then you don't have to release it unless they request. At that point, THEY may do with the code whatever they please.
But you can choose which license you want, so if that's a concern to you, leverage the CDDL for Metro for your project instead, it's much more liberal.
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