把nginx的access_log以json的格式输出

# 在`nginx.conf`中添加如下配置
log_format json escape=json '{"@timestamp":"$time_iso8601",'
                      '"server_addr":"$server_addr",'
                      '"remote_addr":"$remote_addr",'
                      '"scheme":"$scheme",'
                      '"request_method":"$request_method",'
                      '"request_uri": "$request_uri",'
                      '"request_length": "$request_length",'
                      '"uri": "$uri",'
                      '"request_time":$request_time,'
                      '"body_bytes_sent":$body_bytes_sent,'
                      '"bytes_sent":$bytes_sent,'
                      '"status":"$status",'
                      '"upstream_time":"$upstream_response_time",'
                      '"upstream_host":"$upstream_addr",'
                      '"upstream_status":"$upstream_status",'
                      '"host":"$host",'
                      '"http_referer":"$http_referer",'
                      '"http_user_agent":"$http_user_agent"'
                      '}';