Good point, but it makes Tracer more difficult to test. A bunch of tests in `TestTraced` and `TestTracer` use a mock of a `TraceRecorder` to check that `record()` is called, and we can't really force sampling there.

https://github.com/wajam/nrv/blob/master/nrv-core/src/test/scala/com/wajam/nrv/tracing/TestTraced.scala#L63