metabase.Open() now reports metabase mode metric. shard.UpdateID()
needs to read shard ID from metabase => needs to open metabase.
It caused reporting 'shard undefined' metrics. To avoid reporting
wrong metrics metabase.GetShardID() was added which also opens
metabase and does not report metrics.
Signed-off-by: Ekaterina Lebedeva <ekaterina.lebedeva@yadro.com>