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

3 files changed, 33 insertions, 1 deletions

diff --git a/internal/rpn/operations_interfaces.go b/internal/rpn/operations_interfaces.go
index 002ec33..6af0973 100644
--- a/internal/rpn/operations_interfaces.go
+++ b/internal/rpn/operations_interfaces.go
@@ -98,6 +98,7 @@ type Operator interface {
 	MetricCategory(stack *Stack, categoryName string) (string, error)
 	MetricCompatible(stack *Stack) (string, error)
 	// Custom metric commands
+	CustomShow(stack *Stack, name string) (string, error)
 	CustomList(stack *Stack) (string, error)
 	CustomDefine(name string, factor float64, category string) error
 	CustomUndefine(name string) error
diff --git a/internal/rpn/operations_metric_cmd.go b/internal/rpn/operations_metric_cmd.go
index 1915507..61e3f82 100644
--- a/internal/rpn/operations_metric_cmd.go
+++ b/internal/rpn/operations_metric_cmd.go
@@ -93,6 +93,33 @@ func parseCategory(name string) (Category, bool) {
 	return 0, false
 }
 
+// CustomShow returns detailed info for custom metrics.
+// If name is empty, shows all custom metrics; otherwise shows the named one.
+func (o *Operations) CustomShow(stack *Stack, name string) (string, error) {
+	reg := o.metricRegistry
+	if name != "" {
+		m, ok := reg.Find(name)
+		if !ok {
+			return "", fmt.Errorf("unknown custom metric %q", name)
+		}
+		if !m.IsCustom {
+			return "", fmt.Errorf("metric %q is not a custom metric", name)
+		}
+		factor := m.Factor(o.GetPrefixMode())
+		return fmt.Sprintf("%s, category: %s, base: %s, factor: %.10g", m.Name, m.Category, m.BaseUnit, factor), nil
+	}
+	metrics := reg.ListByCategory(Custom)
+	if len(metrics) == 0 {
+		return "no custom metrics defined", nil
+	}
+	var lines []string
+	for _, m := range metrics {
+		factor := m.Factor(o.GetPrefixMode())
+		lines = append(lines, fmt.Sprintf("  %s, category: %s, base: %s, factor: %.10g", m.Name, m.Category, m.BaseUnit, factor))
+	}
+	return strings.Join(lines, "\n"), nil
+}
+
 // CustomList returns all custom metric names.
 func (o *Operations) CustomList(stack *Stack) (string, error) {
 	reg := o.metricRegistry
diff --git a/internal/rpn/rpn_parse.go b/internal/rpn/rpn_parse.go
index 615e804..9906f98 100644
--- a/internal/rpn/rpn_parse.go
+++ b/internal/rpn/rpn_parse.go
@@ -575,7 +575,11 @@ func (r *RPN) handleCustomCommand(stack *Stack, tokens []string, i int) (string,
 	subCmd := tokens[i+1]
 	switch subCmd {
 	case "show":
-		result, err := r.ops.MetricShow(stack)
+		name := ""
+		if i+2 < len(tokens) {
+			name = tokens[i+2]
+		}
+		result, err := r.ops.CustomShow(stack, name)
 		if err != nil {
 			return "", true, fmt.Errorf("rpn: custom show: %w", err)
 		}