Add SpanFilter - an option to mutate FinishedSpans

[marcingrzejszczak]

Currently SpanFilter is a global way of filtering out whether a FinishedSpan should be exported or not. We should rename SpanFilter to SpanExportingPredicate to avoid confusion. SpanFilter then would give an option to mutate spans before they are reported to a n external system. We want to keep consistent nomenclature with ObservationFilter and MeterFilter.