node: add metrics for shard's capacity

[End-rey]

Closes #1794.

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 25.71429% with 26 lines in your changes missing coverage. Please review.

Project coverage is 22.87%. Comparing base (2bb903c) to head (14b21ae).

Files with missing lines	Patch %	Lines
cmd/neofs-node/metrics.go	0.00%	21 Missing :warning:
cmd/neofs-node/main.go	0.00%	3 Missing :warning:
pkg/metrics/engine.go	81.81%	2 Missing :warning:

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #3022 +/- ## ========================================== - Coverage 22.87% 22.87% -0.01% ========================================== Files 791 791 Lines 58688 58723 +35 ========================================== + Hits 13425 13433 +8 - Misses 44366 44393 +27 Partials 897 897 ```

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