fix: Update _onSpanFinish when _spanBuffer is updated

[ChrisMcD1]

Without _onSpanFinish updated, then whenever a span is closed, it will will be offered to the old queue, that nothing is reading from anymore. This causes all traces to not be sent.

fixes https://github.com/kamon-io/Kamon/issues/1281

[ivantopo]

Hey @ChrisMcD1, great catch!

[thjaeckle]

@ivantopo is there a plan to provide this fix in a 2.7 bugfix release? We in the @eclipse-ditto project would want to configure this and face issue #1281