tracing: add WithTags() to define default tags (#11)

This PR adds the new `WithTags` TraceOption setting to allow users to
define custom tags that will be added by default to each span.

tracing.go

@@ -27,6 +27,16 @@ type TraceServerHooks struct {
 
 type TraceOptions struct {
 	includeClientErrors bool
+	tags                []TraceTag
+}
+
+// TraceTag represents a single span tag.
+type TraceTag struct {
+	Key string
+
+	// Value defines the span's tag value. Values can be numeric types, strings, or
+	// bools.
+	Value interface{}
 }
 
 type TraceOption func(opts *TraceOptions)
@@ -39,6 +49,14 @@ func IncludeClientErrors(includeClientErrors bool) TraceOption {
 	}
 }
 
+// WithTags defines tags to be added to each outoing span by default.  If there
+// is a pre-existing tag set for `key`, it is overwritten.
+func WithTags(tags ...TraceTag) TraceOption {
+	return func(opts *TraceOptions) {
+		opts.tags = tags
+	}
+}
+
 // NewOpenTracingHooks provides a twirp.ServerHooks struct which records
 // OpenTracing spans.
 func NewOpenTracingHooks(tracer ot.Tracer, opts ...TraceOption) *twirp.ServerHooks {
@@ -86,6 +104,12 @@ func (t *TraceServerHooks) startTraceSpan(ctx context.Context) (context.Context,
 		if serviceName, ok := twirp.ServiceName(ctx); ok {
 			span.SetTag("service", serviceName)
 		}
+
+		if len(t.opts.tags) != 0 {
+			for _, tag := range t.opts.tags {
+				span.SetTag(tag.Key, tag.Value)
+			}
+		}
 	}
 
 	return ctx, nil

tracing_test.go

@@ -21,7 +21,7 @@ func TestTracingHooks(t *testing.T) {
 	tests := []struct {
 		desc         string
 		service      twirptest.Haberdasher
-		traceOpts   []TraceOption
+		traceOpts    []TraceOption
 		expectedTags map[string]interface{}
 		expectedLogs []mocktracer.MockLogRecord
 		errExpected  bool
@@ -38,6 +38,26 @@ func TestTracingHooks(t *testing.T) {
 			},
 			expectedLogs: []mocktracer.MockLogRecord{},
 		},
+		{
+			desc:    "set tags and logs with additional tags",
+			service: twirptest.NoopHatmaker(),
+			traceOpts: []TraceOption{
+				WithTags(
+					TraceTag{"foo", "bar"},
+					TraceTag{"city", "tokyo"},
+				),
+			},
+			expectedTags: map[string]interface{}{
+				"package":          "twirptest",
+				"component":        "twirp",
+				"service":          "Haberdasher",
+				"span.kind":        serverType,
+				"http.status_code": int64(200),
+				"foo":              "bar",
+				"city":             "tokyo",
+			},
+			expectedLogs: []mocktracer.MockLogRecord{},
+		},
 		{
 			desc:    "set tags and logs with operation name for an errored request",
 			service: twirptest.ErroringHatmaker(errors.New("test")),
@@ -68,8 +88,8 @@ func TestTracingHooks(t *testing.T) {
 			errExpected: true,
 		},
 		{
-			desc:       "user error should be not reported as an erroneous span when correct option is set",
-			service:    twirptest.ErroringHatmaker(twirp.NotFoundError("not found")),
+			desc:      "user error should be not reported as an erroneous span when correct option is set",
+			service:   twirptest.ErroringHatmaker(twirp.NotFoundError("not found")),
 			traceOpts: []TraceOption{IncludeClientErrors(false)},
 			expectedTags: map[string]interface{}{
 				"package":          "twirptest",