apm: populate System.Container in metadata

CHANGELOG.md

@@ -5,6 +5,7 @@
  - Stop pooling Transaction/Span/Error, introduce internal pooled objects (#319)
  - Enable metrics collection with default interval of 30s (#322)
  - `ELASTIC_APM_SERVER_CERT` enables server certificate pinning (#325)
+ - Add Docker container ID to metadata (#330)
 
 ## [v1.0.0](https://github.com/elastic/apm-agent-go/releases/tag/v1.0.0)
 

tracer_test.go

@@ -20,6 +20,7 @@ import (
 	"github.com/stretchr/testify/require"
 
 	"go.elastic.co/apm"
+	"go.elastic.co/apm/internal/apmhostutil"
 	"go.elastic.co/apm/transport"
 	"go.elastic.co/apm/transport/transporttest"
 )
@@ -352,6 +353,24 @@ func TestTracerBodyUnread(t *testing.T) {
 	tracer.Flush(nil)
 }
 
+func TestTracerMetadata(t *testing.T) {
+	tracer, recorder := transporttest.NewRecorderTracer()
+	defer tracer.Close()
+
+	tracer.StartTransaction("name", "type").End()
+	tracer.Flush(nil)
+
+	// TODO(axw) check other metadata
+	system, _, _ := recorder.Metadata()
+	containerID, err := apmhostutil.ContainerID()
+	if err != nil {
+		assert.Nil(t, system.Container)
+	} else {
+		require.NotNil(t, system.Container)
+		assert.Equal(t, containerID, system.Container.ID)
+	}
+}
+
 type blockedTransport struct {
 	transport.Transport
 	unblocked chan struct{}

utils.go

@@ -11,6 +11,7 @@ import (
 
 	"github.com/pkg/errors"
 
+	"go.elastic.co/apm/internal/apmhostutil"
 	"go.elastic.co/apm/internal/apmstrings"
 	"go.elastic.co/apm/model"
 )
@@ -74,6 +75,11 @@ func getLocalSystem() model.System {
 		}
 	}
 	system.Hostname = truncateString(system.Hostname)
+	if containerID, err := apmhostutil.ContainerID(); err == nil {
+		system.Container = &model.Container{
+			ID: containerID,
+		}
+	}
 	return system
 }