Add shard free space to metrics

[EESergey]

Is your feature request related to a problem? Please describe.

Required to calculate free space in a cluster.

Describe the solution you'd like

# TYPE neofs_node_engine_freespace gauge
neofs_node_engine_freespace{shard="Jjam495aBJubsKkRXTYqJz"} 1.609826304e+11
neofs_node_engine_freespace{shard="RU5y6bserpbKNLPdXi9rji"} 1.609826304e+11

Describe alternatives you've considered

Add shard mountpoint to metrics.

[roman-khimov]

But you can have it in many other ways from the system, why node should care?

[EESergey]

I only need free space on the data shards, not the write cache or the system disk. I can get the free space from the node_exporter, but then I need mount points.