Add package and instance id to CIPD user agent.

cipd/client/cipd/client.go

Index: cipd/client/cipd/client.go
diff --git a/cipd/client/cipd/client.go b/cipd/client/cipd/client.go
index 53bc31ad3efb18ef453457f29364c06d4a88164f..0d73e8d01b506696b8316749d8042eca098f43d1 100644
--- a/cipd/client/cipd/client.go
+++ b/cipd/client/cipd/client.go
@@ -74,9 +74,6 @@ const (
 	// TagAttachTimeout is how long to wait for an instance to be processed when attaching tags.
 	TagAttachTimeout = 3 * time.Minute
 
-	// UserAgent is HTTP user agent string for CIPD client.
-	UserAgent = "cipd 1.3"
-
 	// ServiceURL is URL of a backend to connect to by default.
 	ServiceURL = "https://chrome-infra-packages.appspot.com"
 )
@@ -110,6 +107,19 @@ var (
 	ErrPackageNotFound = errors.New("no such package")
 )
 
+var (
+	// UserAgent is HTTP user agent string for CIPD client.
+	UserAgent = "cipd 1.3"
+)
+
+func init() {
+	ver, err := version.GetStartupVersion()
+	if err != nil {
+		return
+	}
+	UserAgent += fmt.Sprintf(" (%s@%s)", ver.PackageName, ver.InstanceID)
+}
+
 // UnixTime is time.Time that serializes to unix timestamp in JSON (represented
 // as a number of seconds since January 1, 1970 UTC).
 type UnixTime time.Time