[DocDB] [Snapshots] Include metadata in on-demand snapshots

[vkulichenko]

Jira Link: DB-2708

Description

Currently, we don't properly support the in-cluster snapshot use case. The scenario is the following:

1. Create a snapshot using the create_snapshot command.
2. Make a schema change (for example, DROP TABLE).
3. Restore the snapshot.

The table will not be restored, because snapshots do not include metadata info, so schema changes are not reverted. At the same time, we do include the metadata in snapshots created as a part of a schedule (for PITR).

Metadata should be included in all snapshots, regardless of how they are created. Ultimately, there should not be any difference between a snapshot created on-demand, or a snapshot created as a part of a schedule.

Metadata should be included by default when create_snapshot command is executed. For compatibility purposes, we can add a --exclude_metadata parameter that would force the command to behave as it does currently.