Setting priority for sample sources

[laksono]

In some cases, it is important to have sample-source A to be more priority than sample-source B.
For instance, we want perf-events sample-source to be more priority than PAPI, so that a native event will be handled by perf-event instead of PAPI.

AFAIK, it's the compiler that connects between sample-sources, and there's no clear way to set priorities.