app/vmctl: vm-native - split migration on per-metric basis
`vm-native` mode now splits the migration process on per-metric basis.
This allows to migrate metrics one-by-one according to the specified filter.
This change allows to retry export/import requests for a specific metric and provides a better
understanding of the migration progress.
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Signed-off-by: hagen1778 <roman@victoriametrics.com>
Co-authored-by: hagen1778 <roman@victoriametrics.com>
