feat: implement custom metric commands (define, undefine, list, show)

- Add Unregister() to MetricRegistry for removing custom metrics
- Handle 'custom show', 'custom list', 'custom define', 'custom undefine' in rpn_parse.go evaluate()
- Add CustomList, CustomDefine, CustomUndefine methods to Operations
- Update Operator interface with new custom metric methods
- Add comprehensive tests for all custom metric operations

1 files changed, 4 insertions, 0 deletions

diff --git a/internal/rpn/operations.go b/internal/rpn/operations.go
index d45ffd1..9e5bbca 100644
--- a/internal/rpn/operations.go
+++ b/internal/rpn/operations.go
@@ -165,6 +165,10 @@ type Operator interface {
 	MetricList(stack *Stack) (string, error)
 	MetricCategory(stack *Stack, categoryName string) (string, error)
 	MetricCompatible(stack *Stack) (string, error)
+	// Custom metric commands
+	CustomList(stack *Stack) (string, error)
+	CustomDefine(name string, factor float64, category string) error
+	CustomUndefine(name string) error
 }
 
 // Operations provides operator implementations and stack manipulation.