#!/bin/bash
s='[06/Jan/2021:17:50:09 +0300] "" 10.139.3.194 407 "CONNECT clients5.google.com:443 HTTP/1.1" "" "-" "" 4245 75 "" "" "81" ""'
sed -E 's/(\[[^][]*]|"[^"]*"|\S+)\s+/\1,/g' <<< "$s"