Filtering Processor example is problematic

[alanwest]

The filtering processor example has some problems.

As noted by @hadashiA, SetParentProvider does not not get invoked on the SimpleActivityExportProcessor in the example.

Personally, I don't think a processor is a great solution for doing this. Even though the specification itself suggests that filtering is a good use case to satisfy with a span processor:

SDK MUST allow users to implement and configure custom processors and decorate built-in processors for advanced scenarios such as tagging or filtering.

I'd be more inclined to write my own exporter that does the filtering. For example, you could write an exporter that wrapped the OTLP exporter. Your exporter would do the filtering and forward to the underlying exporter.

I think replacing the example with an example of a custom exporter that does filtering may be a better solution than trying to provide a mechanism to set the provider.

Originally posted by @alanwest in #3201 (comment)