Up to now the log level has been hardcoded at LOG. A new
auto_explain.log_level setting allows that to be modified.

Discussion: https://postgr.es/m/CAPPfruyZh+snR2AdmutrA0B_caj=yWZkLqxUTZYNjJCaQ_wKQg@mail.gmail.com

Tom Dunstan and Andrew Dunstan
Reviewed by Daniel Gustafsson