otel: prevent sampling processor to discard traces that are explicitly selected for tracing

Created by: jhchabran

#41176 added support for the probabilistic sampling processor, but this comes with an issue: if we enable selective tracing and that we explicitly ask for a request to be traced, chances are that the collector will still discard the trace regardless of that flag.

This is counter-intuitive at worse, surprising at best, so it would be great to get this fixed in the future.

The most probable course of action is rely on the sampling.priority tag that we could add on spans to signify that this request should not be discarded. See https://github.com/open-telemetry/opentelemetry-collector-contrib/tree/main/processor/probabilisticsamplerprocessor for more informations about this.