PMM-12361 enable pmm-admin inventory help command (#2393)

* PMM-12361 add RunCmd to pmm-admin inventory

* PMM-12361 remove redundant types

* PMM-12361 fix a typo

* PMM-12361 rephrase the help description

admin/commands/inventory/inventory.go

@@ -23,9 +23,9 @@ import (
 
 // InventoryCommand is used by Kong for CLI flags and commands.
 type InventoryCommand struct {
-	List   ListCommand   `cmd:"" hidden:"" help:"List inventory commands"`
-	Add    AddCommand    `cmd:"" hidden:"" help:"Add to inventory commands"`
-	Remove RemoveCommand `cmd:"" hidden:"" help:"Remove from inventory commands"`
+	List   ListCommand   `cmd:"" help:"List inventory commands"`
+	Add    AddCommand    `cmd:"" help:"Add to inventory commands"`
+	Remove RemoveCommand `cmd:"" help:"Remove from inventory commands"`
 }
 
 // ListCommand is used by Kong for CLI flags and commands.
@@ -105,3 +105,6 @@ func formatTypeValue(acceptableTypeValues map[string][]string, input string) (*s
 	}
 	return nil, errors.Errorf("unexpected type value %q", input)
 }
+
+// RunCmd is a stub that allows to display the InventoryCommand's help.
+func (cmd *InventoryCommand) RunCmd() {}