A Phase 3 Prospective, Randomized, Multicenter, Open-Label, Central Assessor Blinded, Parallel Group, Comparative Study to Determine the Efficacy, Safety and Tolerability of Aztreonam Avibactam (ATM-AVI) ± Metronidazole (MTZ) Versus Meropenem ± Colistin (MER ± COL) for the Treatment of Serious Infections due to Gram-Negative Bacteria, Including Metallo β-Lactamase (MBL) Producing Multidrug Resistant Pathogens, for Which There are Limited or No Treatment Options