Use the correct default values for compaction group configs

[hzxa21]

Currently compaction group configs are persisted in meta store and will be restored on meta restart. We have met issues related to how default values are determined:

• For newly added configs, we currently cannot differentiate default value and missing values. For example, the newly added enable_emergecy_picker and tombstone_recliam_ratio configs on existing compaction groups will be interpreted as false and 0, which can cause unexpected behavior after version upgrade.
• For configs with changed default values, we currently cannot handle as well because we cannot differentiate whether user has explicitly configured the field. For example, the default value of level0_stop_write_threshold_sub_level_number has changed from 1000 to 300 but existing compaction groups won't adopt this default value changes even though user hasn't explicitly configured it.

The solution is quite simple: we should use the optional keyword in compaction group config proto so that we can perform presence check

• If a field is none (absent), use default value
• If a field is set, use the persisted value

[hzxa21]

For newly added fields, we can use optional directly.

For existing fields that use default values, maybe we can do the change altogether with the SQL meta store backend migration.

[hzxa21]

This is more like a guideline instead of a working item. I will remove it from milestone.

[github-actions]

This issue has been open for 60 days with no activity. Could you please update the status? Feel free to continue discussion or close as not planned.