rpn: implement custom show to display custom metric details

Previously, 'custom show' delegated to MetricShow(), which displayed
metric info for the top stack value — identical to 'metric show' and
making 'custom show' a pointless alias.

Now 'custom show' lists all custom metrics with name, category, base
unit, and factor. Optionally accepts a metric name argument to show
details for a specific custom metric only.

- Add CustomShow(stack, name) method to Operator interface
- Implement CustomShow on Operations with per-metric detail output
- Update handleCustomCommand to call CustomShow instead of MetricShow

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