zipkin server middleware: set error tag on http error status codes

tracing/zipkin/http.go

@@ -193,6 +193,10 @@ func HTTPServerTrace(tracer *zipkin.Tracer, options ...TracerOption) kithttp.Ser
 		func(ctx context.Context, code int, r *http.Request) {
 			if span := zipkin.SpanFromContext(ctx); span != nil {
 				zipkin.TagHTTPStatusCode.Set(span, strconv.Itoa(code))
+				if code > 399 {
+					// set http status as error tag (if already set, this is a noop)
+					zipkin.TagError.Set(span, http.StatusText(code))
+				}
 				if rs, ok := ctx.Value(kithttp.ContextKeyResponseSize).(int64); ok {
 					zipkin.TagHTTPResponseSize.Set(span, strconv.FormatInt(rs, 10))
 				}

tracing/zipkin/http_test.go

@@ -2,6 +2,7 @@ package zipkin_test
 
 import (
 	"context"
+	"errors"
 	"fmt"
 	"net/http"
 	"net/http/httptest"
@@ -173,7 +174,7 @@ func TestHTTPServerTrace(t *testing.T) {
 	handler := kithttp.NewServer(
 		endpoint.Nop,
 		func(context.Context, *http.Request) (interface{}, error) { return nil, nil },
-		func(context.Context, http.ResponseWriter, interface{}) error { return nil },
+		func(context.Context, http.ResponseWriter, interface{}) error { return errors.New("dummy") },
 		zipkinkit.HTTPServerTrace(tr),
 	)
 
@@ -214,4 +215,8 @@ func TestHTTPServerTrace(t *testing.T) {
 	if want, have := httpMethod, spans[0].Name; want != have {
 		t.Errorf("incorrect span name, want %s, have %s", want, have)
 	}
+
+	if want, have := http.StatusText(500), spans[0].Tags["error"]; want != have {
+		t.Fatalf("incorrect error tag, want %s, have %s", want, have)
+	}
 }