Jaeger: Make close of migrationSpan more robust if planning to turn on by default in user envs.

[djwhatle]

See #967 (comment)

Closing the final Jaeger span for a migration can cause problems if any other spans are open at the time. A naive solution I wrote waits 5 minutes after the migmigration completes to close the span, giving any ongoing reconciles 5 minutes to return. I think this will cover 99.9% of cases but if we deploy this as something other than a dev tool we should consider making this close action more robust and not allowing any subsequent child span writes.