set the log in nginx.conf

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location /docs {
	keepalive_timeout 0;
	default_type 'text/html';
	set $res "";
	set_by_lua_file $res /opt/openresty/set_by.lua;
	rewrite_by_lua_file /opt/openresty/rewrite.lua;
	access_by_lua_file /opt/openresty/access.lua;
	content_by_lua_file /opt/openresty/content.lua;
	header_filter_by_lua_file /opt/openresty/header_filter.lua;
	body_filter_by_lua_file /opt/openresty/body_filter.lua;
	log_by_lua_file /opt/openresty/base/log.lua;
	proxy_pass http://default_doc_upstream;
}

log_by_lua_file

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local cjson = require("cjson");

local upstream_addr = nil;
if nil ~= ngx.var.upstream_addr then
    upstream_addr = ngx.var.upstream_addr;
else
    upstream_addr = nil;
end

ngx.log(ngx.INFO, "upstream_addr="..upstream_addr);

--
if nil ~= ngx.var.upstream_response_time and 'number' == type(ngx.var.upstream_response_time) then
    local resp_time_so_far = ngx.now() - tonumber(ngx.var.upstream_response_time);
    -- 1s
    if tonumber(ngx.var.upstream_response_time) >= 1 then
        ngx.log(ngx.WARN, "[SLOW] request_time="..ngx.var.request_time.." upstream_response_time="..ngx.var.upstream_response_time..", upstream_addr="..upstream_addr);
    end
end