fix user metrics in @OnTimer

[liferoad]

Fixes https://github.com/apache/beam/issues/29099

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[scwhittle]

@liferoad I don't think this is needed. I added a test in https://github.com/apache/beam/pull/35494 and counter plumbing appears to be working without any change.

That test also verifies that finishBundle is called which would flush the state. I'm not sure either how the state flushing would be tied to user counters.

[liferoad]

Sounds good. Thanks for checking this. I will also close the issue.