prune metrics for DDPG:
I think the right way to go is to use a small set of metrics. Otherwise, DDPG can hardly to learn. Therefore, I pick dba_hist_sys_time_model, which can represent the workload and only has 31 metrics.
clean up convert_dbms_metrics:
remove unused base_metric_data; rename metric_data to numeric_metric_data; use all metric_data instead of numeric_metric_data to calculate target objectives.
fix invalid results handling bug:
check if the last_conf is None before copying that to the knob file.