Efficient fluorescence collection is essential for precise quantum measurements based on nitrogen-vacancy (NV) centers in diamond. However, photon extraction is limited by total internal reflection at the diamond-air interface. To address this challenge, we propose a short-focal-length Si3N4 metalens on a SiO2 support for broadband fluorescence collection over 600-800 nm in an ensemble-NV ODMR system. The metalens is designed as a compact replacement for the conventional lens collection path. FDTD simulations show that the metalens improves the forward photon collection efficiency by a factor of 3.2 compared with bare diamond. Experimentally, ODMR measurements show fluorescence-related signal enhancements of 33% and 45% for micro-ring and double-split-ring microwave antennas, respectively. These results confirm that the proposed design provides an effective and compact solution for enhancing fluorescence collection in diamond-based quantum sensing systems.