TracesSampler does not contain a path/name

Environment

How do you use Sentry? Sentry SaaS (sentry.io)

Which SDK and version? Sentry 3.0.3, .Net 3.1

Steps to Reproduce

I just upgraded to 3.0.3 and was going to use the performance monitor feature. It worked great, but unfortunately not the TracesSampler that I wanted to use to filter out calls to /healthcheck. The documentation is not correct because the CustomSamplingContext is not a string as in the example here: https://docs.sentry.io/platforms/dotnet/performance/sampling/.

I used the following code in Program.cs. The transaction name seems to be prefixed with GET and I couldn’t find a property with just the path.

options.TracesSampler = context =>
{
    var ctx = context.TransactionContext;
    return ctx.Name switch
    {
        // The health check endpoint is just noise - drop all transactions
        "GET /healthcheck" => 0.0,

        // Default sample rate
        _ => null
    };
};

That would still have been fine, but in my case the name was Unknown Route. It seems the transaction name is set after the TracesSampler is called. The code in Startup.cs file looks something like below.

app.UseRouting();
app.UseSentryTracing();
app.UseCors();

app.UseEndpoints(endpoints =>
{
    endpoints.Map("/healthcheck", _ => Task.CompletedTask);
    endpoints.MapControllers();
});

If I move UseSentryTracing to below UseEndpoints it doesn’t call TracesSampler at all.

Am I missing something or is it a bug?

Expected Result

I expected TracesSampler to be called with a context that contains the path of the call in order to avoid sending transactions that is not relevant.

Actual Result

TracesSampler was called with a context where the transaction name was Unknown route.

[bruno-garcia]

Sounds like we should add this to the docs @Tyrrrz

[Tyrrrz]

That should be possible to do with dynamic sampling already, I think. You can implement it in such way that it counts how many times it was called in the past minute and either returns 1.0 (sample in) or 0.0 (sample out). Something like this:

UseSentry(options =>
{
     var lastTransanctionTimestamp = DateTimeOffset.MinValue;
     var transactionCount = 0;

     options.TracesSampler = ctx =>
     {
          var now = DateTimeOffset.Now;
          if (now.Minute != lastTransanctionTimestamp.Minute || now - lastTransanctionTimestamp > TimeSpan.FromMinutes(1))
          {
                transactionCount = 0;
          }

          var sampleRate = transactionCount < 50 ? 1.0 : 0.0;
          
          lastTransanctionTimestamp = now;
          transactionCount++;

          return sampleRate;
     }
})