The paper reports on the realization of a metropolitan-scale quantum link for generating heralded entanglement between two independently operated quantum network nodes separated by 10 kilometers. The nodes, each hosting a diamond spin qubit, are connected via 25 kilometers of optical fiber. To minimize fiber photon loss, quantum frequency conversion of the qubit-native photons to the telecom L-band is used, and an extensible phase-stabilized architecture is employed. The architecture enables independent operation of the nodes, mitigates photon loss, and supports full heralding of entanglement generation. The performance of the system is benchmarked using parameter monitoring and postselected entanglement generation experiments. The fully heralded entanglement generation demonstrates the delivery of entangled states ready for further use, marking a significant milestone in the development of large-scale quantum networking. The architecture is compatible with different qubit systems and can be extended to connect more nodes, paving the way for advanced quantum network protocols and applications.The paper reports on the realization of a metropolitan-scale quantum link for generating heralded entanglement between two independently operated quantum network nodes separated by 10 kilometers. The nodes, each hosting a diamond spin qubit, are connected via 25 kilometers of optical fiber. To minimize fiber photon loss, quantum frequency conversion of the qubit-native photons to the telecom L-band is used, and an extensible phase-stabilized architecture is employed. The architecture enables independent operation of the nodes, mitigates photon loss, and supports full heralding of entanglement generation. The performance of the system is benchmarked using parameter monitoring and postselected entanglement generation experiments. The fully heralded entanglement generation demonstrates the delivery of entangled states ready for further use, marking a significant milestone in the development of large-scale quantum networking. The architecture is compatible with different qubit systems and can be extended to connect more nodes, paving the way for advanced quantum network protocols and applications.