LogsSink/TracesSink/MetricsSink should offer a way to allow waiting until they receive data

[atoulme]

Is your feature request related to a problem? Please describe.
Right now, we see a lot of flaky tests which end being up related to the asynchronous nature of waiting for a sink to receive test data so we can test it.

Describe the solution you'd like
Rather than use time.Sleep in tests, offer a way to use a sync.WaitGroup or another mechanism similar to allow consumers to react to the fact that the sink received data.

Describe alternatives you've considered
We can implement a separate sink class for this, or a wrapper for the sink.