Go: add stats (#17061)

### Summary

Add LLM token stats framework

---------

Signed-off-by: Jin Hai <haijin.chn@gmail.com>

internal/common/model_type.go

@@ -0,0 +1,51 @@
+//
+//  Copyright 2026 The InfiniFlow Authors. All Rights Reserved.
+//
+//  Licensed under the Apache License, Version 2.0 (the "License");
+//  you may not use this file except in compliance with the License.
+//  You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+//  Unless required by applicable law or agreed to in writing, software
+//  distributed under the License is distributed on an "AS IS" BASIS,
+//  WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+//  See the License for the specific language governing permissions and
+//  limitations under the License.
+//
+
+package common
+
+import (
+	"fmt"
+	"time"
+)
+
+type ModelUsage struct {
+	UserID         string    `json:"user_id"`
+	UserEmail      string    `json:"user_email"`
+	TenantID       string    `json:"tenant_id"`
+	TenantEmail    string    `json:"tenant_email"`
+	GroupID        string    `json:"group_id"`
+	GroupName      string    `json:"group_name"`
+	AppID          string    `json:"app_id"`
+	AppName        string    `json:"app_name"`
+	SessionID      string    `json:"session_id"`
+	ProviderName   string    `json:"provider_name"`
+	InstanceID     string    `json:"instance_id"`
+	APIKey         string    `json:"api_key"`
+	ModelName      string    `json:"model_name"`
+	Type           string    `json:"type"`
+	RequestID      string    `json:"request_id"`
+	StartAt        time.Time `json:"start_at"`
+	ResponseTimeMS int64     `json:"response_time_ms"`
+	ErrorMessage   string    `json:"error_message"`
+
+	InputTokens  int `json:"input_tokens"`
+	OutputTokens int `json:"output_tokens"`
+	TotalTokens  int `json:"total_tokens"`
+}
+
+func (m *ModelUsage) String() string {
+	return fmt.Sprintf("%#v", m)
+}