Support trace information in logs

README.md

@@ -55,6 +55,7 @@ to use the log details in the Stackdriver monitoring interface.
 * [`Label`](#label)
 * [`SourceLocation`](#sourcelocation)
 * [`Operation`](#operation)
+* [`TraceContext`](#tracecontext)
 
 #### HTTP
 
@@ -206,6 +207,21 @@ OperationCont(id, producer string) zap.Field
 OperationEnd(id, producer string) zap.Field
 ```
 
+#### TraceContext
+
+You can add trace context information to your log lines to be picked up by
+Stackdriver.
+
+```golang
+TraceContext(trace string, spanId string, sampled bool, projectName string) []zap.Field
+```
+
+Like so:
+
+```golang
+logger.Error("Something happened!", zapdriver.TraceContext("105445aa7843bc8bf206b120001000", "0", true, "my-project-name")...)
+```
+
 ### Pre-configured Stackdriver-optimized encoder
 
 The Stackdriver encoder maps all Zap log levels to the appropriate

trace.go

@@ -0,0 +1,24 @@
+package zapdriver
+
+import (
+	"fmt"
+	"go.uber.org/zap"
+	"strconv"
+)
+
+const (
+	traceKey = "logging.googleapis.com/trace"
+	spanKey = "logging.googleapis.com/spanId"
+	traceSampledKey = "logging.googleapis.com/trace_sampled"
+)
+
+// TraceContext adds the correct Stackdriver "trace", "span", "trace_sampled fields
+//
+// see: https://cloud.google.com/logging/docs/reference/v2/rest/v2/LogEntry
+func TraceContext(trace string, spanId string, sampled bool, projectName string) []zap.Field {
+	return []zap.Field{
+		zap.String(traceKey, fmt.Sprintf("projects/%s/traces/%s", projectName, trace)),
+		zap.String(spanKey, spanId),
+		zap.String(traceSampledKey, strconv.FormatBool(sampled)),
+	}
+}

trace_test.go

@@ -0,0 +1,18 @@
+package zapdriver
+
+import (
+	"github.com/stretchr/testify/assert"
+	"go.uber.org/zap"
+	"testing"
+)
+
+func TestTraceContext(t *testing.T) {
+	t.Parallel()
+
+	fields := TraceContext("105445aa7843bc8bf206b120001000", "0", true, "my-project-name")
+	assert.Equal(t, fields, []zap.Field{
+		zap.String(traceKey, "projects/my-project-name/traces/105445aa7843bc8bf206b120001000"),
+		zap.String(spanKey, "0"),
+		zap.String(traceSampledKey, "true"),
+	})
+}