Add ability to filter and defer spans

[sigix]

Signed-off-by: Edward Bross edward.bross@appian.com

Which problem is this PR solving?

• Resolves Deferred span reporting #815

Short description of the changes

• adds the ability to reduce the number of spans reported by setting a filter and a defer threshold
• implemented via a new FilteringReporter that handles the filtering and buffering logic before forwarding spans to a delegate

[yurishkuro]

I prefer not to merge this because this sort of naive filtering can easily lead to broken traces, it is not particularly general as a mechanism, and in the current form can be implemented as an extension instead of being part of the core lib.

Cc @pavolloffay @jpkrohling - thoughts?

[jpkrohling]

There's some discussion about filtering in general here: open-telemetry/opentelemetry-collector-contrib#6341

In this particular case, I see how it could be useful to filter out spans quicker than a specific threshold, but I have the feeling that this is too little: users will want a way to filter spans based on other characteristics. According to our deprecation plan, we are not accepting new features in about one week, so, there won't be a way to do any other type of filtering.

If this comes with good and clear documentation that this is useful to specific use cases and that there are known side-effects (perhaps with concrete examples), I'd be OK in accepting this feature. Otherwise, I think it might cause more harm than good.

[sigix]

@jpkrohling I added a section to jaeger-core README.md with documentation and a use case.

@yurishkuro - using OpenTelemetry, I can see how to implement this via extension instead of modification, but it was not evident to me how to do the same with this library -- the configuration of the reporter seemed to be completely internal to the Configuration class with no accessible extension points. If there is a way to do this via extension, that would be just fine.

[yurishkuro]

There's no configuration based extension mechanism, but you can always instantiate a custom reporter and pass it to the tracer.