Support Datadog tracer priority sampling

old/middlewares/tracing/datadog/datadog.go

@@ -18,6 +18,7 @@ type Config struct {
 	LocalAgentHostPort string `description:"Set datadog-agent's host:port that the reporter will used. Defaults to localhost:8126" export:"false"`
 	GlobalTag          string `description:"Key:Value tag to be set on all the spans." export:"true"`
 	Debug              bool   `description:"Enable DataDog debug." export:"true"`
+	PrioritySampling   bool   `description:"Enable priority sampling. When using distributed tracing, this option must be enabled in order to get all the parts of a distributed trace sampled."`
 }
 
 // Setup sets up the tracer
@@ -29,12 +30,16 @@ func (c *Config) Setup(serviceName string) (opentracing.Tracer, io.Closer, error
 		value = tag[1]
 	}
 
-	tracer := ddtracer.New(
+	opts := []datadog.StartOption{
 		datadog.WithAgentAddr(c.LocalAgentHostPort),
 		datadog.WithServiceName(serviceName),
 		datadog.WithGlobalTag(tag[0], value),
 		datadog.WithDebugMode(c.Debug),
-	)
+	}
+	if c.PrioritySampling {
+		opts = append(opts, datadog.WithPrioritySampling())
+	}
+	tracer := ddtracer.New(opts...)
 
 	// Without this, child spans are getting the NOOP tracer
 	opentracing.SetGlobalTracer(tracer)