Do you need average response time by a field in the events or average of all response times in entire data set ? because if you ultimately require table of raw data(json) and average response time then it should be by some field in your events.
You can do like this :
your base search | rex "response_Time\=\"(?<response_time>\d+)\"" | table _raw response_time | eventstats avg(response_time) as average
You can add any field after the by clause. Let me know if it works, i can help further.