Add debug msg for empty enum metric

[gsanchezgavier]

This adds a debug message for the case that all enum metrics has value 0

windows_service_start_mode{name="ktmrm",start_mode="auto"} 0 windows_service_start_mode{name="ktmrm",start_mode="boot"} 0 windows_service_start_mode{name="ktmrm",start_mode="disabled"} 0 windows_service_start_mode{name="ktmrm",start_mode="manual"} 0 windows_service_start_mode{name="ktmrm",start_mode="system"} 0