feat: usage struct add CompletionTokensDetails (#863)

common.go

@@ -7,4 +7,10 @@ type Usage struct {
 	PromptTokens            int                      `json:"prompt_tokens"`
 	CompletionTokens        int                      `json:"completion_tokens"`
 	TotalTokens             int                      `json:"total_tokens"`
+	CompletionTokensDetails *CompletionTokensDetails `json:"completion_tokens_details"`
+}
+
+// CompletionTokensDetails Breakdown of tokens used in a completion.
+type CompletionTokensDetails struct {
+	ReasoningTokens int `json:"reasoning_tokens"`
 }