repair: Add progress metrics for bootstrap ops
repair: Add progress metrics for bootstrap ops

The following metric is added:

scylla_node_maintenance_operations_bootstrap_finished_percentage{shard="0",type="gauge"} 0.850000

It is the number of finished percentage for bootstrap operation so far.

Fixes #1244, #6733

avikivity/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla,scylladb/scylla,avikivity/scylla,scylladb/scylla