Get end user usage stats

curl --request GET \
  --url https://app.usechar.ai/api/usage/end-users/{end_user_id}/stats \
  --header 'Authorization: Bearer <token>'

{
  "total_requests": 123,
  "total_prompt_tokens": 123,
  "total_completion_tokens": 123,
  "total_tokens": 123
}