feat: add event-counting filtering layer for spans #228
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Motivation
Applications can generate a large amount of logs which may be handled independently from tracing span events.
Simply filtering with a per-layer-filter will however lose the information that there may be more data regarding the spans elsewhere in the system.
Solution
Add a layer that filters out events but keeps a running count of them. This can then be used when viewing a trace to identify which spans have more contextual information in other systems.