Add log/fields helpers for keys from specification

Opentracing spec specify prefered names for log messages [1]
This patch adds declaration for fields which are meaningful for golan

- log: add helpers function for spec keys
- ext: add LogError() helper for error

LogError() helper can not be declarated in log package because it depends
opentrace.Span which result in cyclic depencency

Footnotes:
[1] https://github.com/opentracing/specification/blob/master/semantic_conventions.md#log-fields-table

ext/field.go

@@ -0,0 +1,17 @@
+package ext
+
+import (
+	"github.com/opentracing/opentracing-go"
+	"github.com/opentracing/opentracing-go/log"
+)
+
+// LogError add error event record for the Span
+func LogError(span opentracing.Span, err error, fields ...log.Field) {
+	Error.Set(span, true)
+	ef := []log.Field{
+		log.Event("error"),
+		log.Error(err),
+	}
+	ef = append(ef, fields...)
+	span.LogFields(ef...)
+}

ext/field_test.go

@@ -0,0 +1,44 @@
+package ext_test
+
+import (
+	"fmt"
+	"reflect"
+	"testing"
+
+	"github.com/stretchr/testify/assert"
+
+	"github.com/opentracing/opentracing-go/ext"
+	"github.com/opentracing/opentracing-go/mocktracer"
+)
+
+func TestLogError(t *testing.T) {
+	tracer := mocktracer.New()
+	span := tracer.StartSpan("my-trace")
+	ext.Component.Set(span, "my-awesome-library")
+	ext.SamplingPriority.Set(span, 1)
+	err := fmt.Errorf("My error")
+	ext.LogError(span, err)
+
+	span.Finish()
+
+	rawSpan := tracer.FinishedSpans()[0]
+	assert.Equal(t, map[string]interface{}{
+		"component": "my-awesome-library",
+		"error":     true,
+	}, rawSpan.Tags())
+
+	assert.Equal(t, len(rawSpan.Logs()), 1)
+	fields := rawSpan.Logs()[0].Fields
+	assert.Equal(t, []mocktracer.MockKeyValue{
+		{
+			Key:         "event",
+			ValueKind:   reflect.String,
+			ValueString: "error",
+		},
+		{
+			Key:         "error",
+			ValueKind:   reflect.String,
+			ValueString: err.Error(),
+		},
+	}, fields)
+}

log/field.go

@@ -143,6 +143,21 @@ func Object(key string, obj interface{}) Field {
 	}
 }
 
+// Event adds a string-valued field with name "value" to a Span.LogFields() record
+func Event(val string) Field {
+	return String("event", val)
+}
+
+// Message adds a string-valued field with name "event" to a Span.LogFields() record
+func Message(val string) Field {
+	return String("message", val)
+}
+
+// Stack adds a string-valued field with name "stack" to a Span.LogFields() record
+func Stack(val string) Field {
+	return String("stack", val)
+}
+
 // LazyLogger allows for user-defined, late-bound logging of arbitrary data
 type LazyLogger func(fv Encoder)
 

log/field_test.go

@@ -34,6 +34,18 @@ func TestFieldString(t *testing.T) {
 			field:    Noop(),
 			expected: ":<nil>",
 		},
+		{
+			field:    Event("test"),
+			expected: "event:test",
+		},
+		{
+			field:    Message("test2"),
+			expected: "message:test2",
+		},
+		{
+			field:    Stack("test3"),
+			expected: "stack:test3",
+		},
 	}
 	for i, tc := range testCases {
 		if str := tc.field.String(); str != tc.expected {