Remove UseDatadog from tracing config (#272)

This was introduced in #247 to allow applications to
use the datadog tracing api directly instead of an opentracing compatible version.

The issue is that when `UseDatadog` is enabled, opentracing.GlobalTracer is set to a noop tracer
so other packages in netlify-commons that emit traces (e.g router) don't work as expected.

We don't actually need this option because `opentracer.New` already calls `tracer.Start` so
applications should be able to use both the opentracing and datadog tracing api.

tracing/config.go

@@ -21,7 +21,6 @@ type Config struct {
 	Port        string `default:"8126"`
 	Tags        map[string]string
 	EnableDebug bool `default:"false" split_words:"true" mapstructure:"enable_debug" json:"enable_debug" yaml:"enable_debug"`
-	UseDatadog  bool `default:"false" split_words:"true" mapstructure:"use_datadog" json:"use_datadog" yaml:"use_datadog"`
 }
 
 func Configure(tc *Config, log logrus.FieldLogger, svcName string) {
@@ -35,12 +34,6 @@ func Configure(tc *Config, log logrus.FieldLogger, svcName string) {
 			tracer.WithLogger(debugLogger{log.WithField("component", "opentracing")}),
 		}
 
-		if tc.UseDatadog {
-			tracerOps = append(tracerOps, tracer.WithLogger(debugLogger{log.WithField("component", "datadog")}))
-		} else {
-			tracerOps = append(tracerOps, tracer.WithLogger(debugLogger{log.WithField("component", "opentracing")}))
-		}
-
 		var serviceTagSet bool
 		for k, v := range tc.Tags {
 			if strings.ToLower(k) == "service" {
@@ -54,11 +47,7 @@ func Configure(tc *Config, log logrus.FieldLogger, svcName string) {
 			tracerOps = append(tracerOps, tracer.WithGlobalTag("service", svcName))
 		}
 
-		if tc.UseDatadog {
-			tracer.Start(tracerOps...)
-		} else {
-			t = opentracer.New(tracerOps...)
-		}
+		t = opentracer.New(tracerOps...)
 	}
 	opentracing.SetGlobalTracer(t)
 }