Enable adding of "rare events"

[JanKrivanek]

Context

We might want to collect info about 'black swan' patterns we understand as very rare and we might be deciding specific changes based on assumed usage of those. Collecting such events with higher sample rate (up to 1) might be helpful to contradict our theories (it might not be used as clear confirmation though - as there might be usages with opted-out telemetry collection).

Suggestion

We should thing about adding internal API that would allow collection of such events from within main or worker nodes without much boilerplate code.
Something along the lines of:

void MSBuildTelemetry.AddEvent(string key, string value, bool needsHashing = true, double samplingRate = 1.0);

The infra would then take care about transfering the event and attaching to the Activity (and overriding the default activity sampling if needed).

The proper design is however needed for this.