[#1055] blobstor: fix mode metric

It used to always show CLOSED after setting shard mode
to read-only regardless of actual mode.
Now metric represents actual blobstor mode of operations.

Signed-off-by: Ekaterina Lebedeva <ekaterina.lebedeva@yadro.com>

pkg/local_object_storage/blobstor/mode.go

@@ -31,5 +31,6 @@ func (b *BlobStor) SetMode(m mode.Mode) error {
 	}
 
 	b.mode = m
+	b.metrics.SetMode(m.ReadOnly())
 	return nil
 }