ext: Add LogFields 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

Footnotes:
[1] https://github.com/opentracing/specification/blob/master/semantic_conventions.md

ext/fields.go

@@ -0,0 +1,31 @@
+package ext
+
+import (
+	"github.com/opentracing/opentracing-go"
+	"github.com/opentracing/opentracing-go/log"
+)
+
+// These constants define common logFields names recommended for better portability across
+// tracing systems and languages/platforms
+// More info: https://github.com/opentracing/specification/blob/master/semantic_conventions.md
+var (
+	LogEvent   = stringLogName("event")
+	LogMessage = stringLogName("message")
+	LogStack   = stringLogName("stack")
+
+	// Span event represent error
+	ErrorEvent = log.String(string(LogEvent), "error")
+)
+
+// Add error event info
+func LogError(span opentracing.Span, err error) {
+	Error.Set(span, true)
+	span.LogFields(ErrorEvent, log.String(string(LogMessage), err.Error()))
+}
+
+// Add log string to to the `span`
+func (logName stringLogName) Log(span opentracing.Span, value string) {
+	span.LogFields(log.String(string(logName), value))
+}
+
+type stringLogName string

ext/fields_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:         "message",
+			ValueKind:   reflect.String,
+			ValueString: err.Error(),
+		},
+	}, fields)
+}