5G mobile communication systems will need to accommodate a variety of use-cases, resulting in a diverse set of requirements. To meet these requirements, 5G systems take advantage of modern virtualization possibilities offered by Network Function Virtualization (NFV), enabling deployment agility and dynamicity of virtualized network functions. With the transformation of telecom towards virtualized environments, advanced observability possibilities gain increasing importance as one of the essential prerequisites, especially for successful DevOps operations. However, deployment agility also puts specific requirements on monitoring solutions in order to adapt automatically and continuously to frequent changes in service deployments. In this short-paper, we establish and discuss essential properties of observability systems for virtual network functions in a 5G context. We take these properties as guiding design principles for our software-defined monitoring framework and outline how to evolve our existing components towards a flexible, scalable, and programmable observability solution for microservice-based NFV with features for increased manageability.