store metric_names in the DB

[lyuyangh]

blocked by #491
part of #433
Stores metric_names in the DB because it is required when we use update_overall_statistics() to update the analysis. update_overall_statistics() does not depend on metadata anymore. It still depends on system_output_data though. I will remove that dependency in the next PR.

Compatibility

• DB migration required
• A new field is added to the returned System model. This is compatible with old versions of explainaboard_api_client so it shouldn't break anything. I tested explainaboard_api_client=0.2.11 (which is the oldest version we support) with this code and it worked OK.

[neubig]

Looks great, thanks!