feat(vertexai/genai): add WithClientInfo option (googleapis#10535)

Add an option that adds a key-value pair to the client info header
of requests from the client.

vertexai/genai/client.go

@@ -88,7 +88,11 @@ func setGAPICClient[ClientType sgci](ctx context.Context, pf *ClientType, conf c
 	if err != nil {
 		return err
 	}
-	c.SetGoogleClientInfo("gccl", internal.Version)
+	kvs := []string{"gccl", internal.Version}
+	if conf.ciKey != "" && conf.ciValue != "" {
+		kvs = append(kvs, conf.ciKey, conf.ciValue)
+	}
+	c.SetGoogleClientInfo(kvs...)
 	*pf = c
 	return nil
 }

vertexai/genai/option.go

@@ -25,13 +25,36 @@ func WithREST() option.ClientOption {
 	return &withREST{}
 }
 
+type withREST struct {
+	internaloption.EmbeddableAdapter
+}
+
 func (w *withREST) applyVertexaiOpt(c *config) {
 	c.withREST = true
 }
 
+// WithClientInfo is an option that sets request information
+// identifying the product that is calling this client.
+func WithClientInfo(key, value string) option.ClientOption {
+	return &withClientInfo{key: key, value: value}
+}
+
+type withClientInfo struct {
+	internaloption.EmbeddableAdapter
+	key, value string
+}
+
+func (w *withClientInfo) applyVertexaiOpt(c *config) {
+	c.ciKey = w.key
+	c.ciValue = w.value
+}
+
 type config struct {
 	// withREST tells the client to use REST as the underlying transport.
 	withREST bool
+	// key-value pair to add to the Google client info header.
+	ciKey   string
+	ciValue string
 }
 
 // newConfig generates a new config with all the given
@@ -51,7 +74,3 @@ type vertexaiClientOption interface {
 	option.ClientOption
 	applyVertexaiOpt(*config)
 }
-
-type withREST struct {
-	internaloption.EmbeddableAdapter
-}