fix: gemini usage (close #354)

relay/channel/gemini/dto.go

@@ -59,4 +59,11 @@ type GeminiChatPromptFeedback struct {
 type GeminiChatResponse struct {
 	Candidates     []GeminiChatCandidate    `json:"candidates"`
 	PromptFeedback GeminiChatPromptFeedback `json:"promptFeedback"`
+	UsageMetadata  GeminiUsageMetadata      `json:"usageMetadata"`
+}
+
+type GeminiUsageMetadata struct {
+	PromptTokenCount     int `json:"promptTokenCount"`
+	CandidatesTokenCount int `json:"candidatesTokenCount"`
+	TotalTokenCount      int `json:"totalTokenCount"`
 }