fix: input flag required for metrics trim command

[harp-intel]

This pull request updates the required flag validation for the trim command in cmd/metrics/trim.go. Instead of relying on Cobra's MarkFlagRequired, the code now explicitly checks if the --input flag was provided and returns a custom error if not.

Flag validation improvements:

• Removed the use of trimCmd.MarkFlagRequired for the --input flag, eliminating reliance on Cobra's built-in required flag mechanism.
• Added an explicit check in validateTrimFlags to ensure that the --input flag is provided, returning a custom error message if it is missing.