tracing: fix context cancellation during shutdown

[LukeCarrier]

tracing.Shutdown() must be invoked before done(), else the context will be cancelled before it's invoked and the following error will be emitted:

E0130 14:31:04.963323       1 tracing.go:156] "failed to shutdown the trace exporter" err="context canceled"

Interestingly, not observed when running with a deschedulingInterval, but reliably reproducible when running as a CronJob.

Nope, I do see it, I just also see metrics that have already been flushed.

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[k8s-ci-robot]

Hi @LukeCarrier. Thanks for your PR.

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