Custom sampler useless for client requests using akka-http

[pnerg]

I just realised there is support in kamon 2.x for providing a custom Samplerimplementation just by configuring it

sampler = "com.area51.CustomSampler"

A bit of hidden feature as it doesn't seem to be documented anywhere....:)

Anyways my issue is that the sampler trait only provides the operation name to use as decision point which is kind of pointless for akka-http client operations as the name contains the actual HTTP operation e.g. GET, POST,...

A simple way to test this:
First a custom sampler

class CustomSampler extends Sampler {
  override def decide(operation: Sampler.Operation): Trace.SamplingDecision = {
    println("sample? -> "+operation.operationName())
    Trace.SamplingDecision.Sample
  }
}

Then a simple akka-http route

private lazy val route: Route =
   pathPrefix("health") {
     get {
       complete(HttpEntity(ContentTypes.`text/plain(UTF-8)`, "OK!"))
     }
   }

Using the akka-http client like this

Http().singleRequest(HttpRequest(uri = s"http://localhost:$port/health")) 

Will render a printout like

sample? -> GET

If I direct a browser towards my local app I instead get a printout like

sample? -> /health

So on the server side of the sampling decision I get the URL which makes it much more usable.
Having the custom sampler on the client side is useless as I can't make any decision based purely on the HTTP operation.

The use cases I'm trying to solve_

• apps periodically poll Consul for config and service updates - These lookups URL's I don't want sampled at all.
• An orchestration system periodically polls a /healthURL on the apps - These I don't want sampled either

Yes there's the new AdaptiveSampler but I want a random sampler with a filtering capability.
This because I want to be able to control the sampling density in runtime just by tweaking probability setting AND have the filtering capability.
Could have been solved in a custom sampler provided I would get more relevant information to make the decision on.

[pnerg]

Hmm, reading the kamon-akka-http code/config I see where the operation name comes from.
Adding a custom name generator could solve this for me but the Sampler trait could perhaps be more useful if provided with more relevant information.

As I browsed the kamon-core code I came across the pre/post hooks which somewhat overlaps with the Sampler as one can mutate the entire span builder.

I'll need to experiment with a combination of custom name generator, sampler and pre-start hook.
The latter to add custom metric tags to certain spans