Sample traces at one rate, but within kept traces, drop specific spans at a higher rate. Can be confusing, but can also sometimes be useful.

subsample_spans.go

const (
     // beeline pre-sampling
     shepherdOverallSampleRate = 3
     triadSampleRate           = 4
)

     ... initialization ...
     // override the default sampler for the beeline
     trace.GlobalConfig.SamplerHook = a.MakeSampler()

// MakeSampler manaages the beeline sampling for shepherd instrumentation. It
// will drop all traces at 1/2 and drop 75% of the per-event pairs of
// verify_schema and pass_to_kafka_writer
func (a *App) MakeSampler() func(map[string]interface{}) (bool, int) {

     a.sampler, _ = sample.NewDeterministicSampler(shepherdOverallSampleRate)

     return func(fields map[string]interface{}) (bool, int) {
             if !a.sampler.Sample(fields["trace.trace_id"].(string)) {
                     return false, shepherdOverallSampleRate
             }
             // we should keep this trace.
             // but if this event is one of the triad we should drop it at 1/4 (on top of the overall rate)
             switch fields["name"] {
             case "verify_schema", "pass_to_kafka_writer":
                     return rand.Intn(triadSampleRate) == 0, triadSampleRate * shepherdOverallSampleRate
             default:
                     return true, shepherdOverallSampleRate
             }
     }

}