[Jaeger] How to support sampling

[simitt]

When using probabilistic sampling Jaeger only sends sampled spans. Elastic APM UI currently requires a 100% of the transaction events. Elastic APM agent only sample spans.
Investigate how to solve sampling with Jaeger integration.

Part of #2886

UPDATE:
As probabilistic sampling support was added with #3487 this Issue now solely focuses on changes necessary to integrate Jaeger sampled traces into the APM app.

[jalvz]

Has there been any thought / discussion about this already?

[axw]

Copying this over from #2886 (comment):

[concept]: When using probabilistic sampling jaeger only sends sampled spans. Elastic APM requires a 100% of the transaction events and only samples spans. Investigate how to solve sampling with jaeger integration.

Outside of the context of Jaeger, there's been some discussion around relying on metrics so we don't need to send and store non-sampled transactions. This is elastic/apm#104.

If we were to do that, we could have the server aggregate metrics based on sampled Jaeger trace data. Jaeger sends up the sampling rate in effect at the time a trace is captured, so we can scale the count/total based on that.

[simitt]

As part of this Issue #3487 was created to add a gRPC Jaeger sampling endpoint. This issue does not consider any changes related to how APM UI calculates transactions per minutes, etc.