The streaming of high quality and omnidirectional videos in Over-the-top (OTT) environments still faces many challenges. The research community is devoting efforts on devising optimized solutions for a variety of key aspects, such as encoding efficiency, Field-of-View (FoV) based streaming, adaptive quality switching, use of network assisted elements, etc. All these initiatives share a common denominator: it is essential to monitor relevant Quality of Service (QoS) and Quality of Experience (QoE) related metrics to better understand what are the limitations, propose appropriate solutions and corroborate the obtained performance. In this context, this paper presents a modular testbed to monitor and register Key Performance Indicators (KPIs) in (omnidirectional) video streaming scenarios, making use of the increasingly adopted Dynamic Adaptive Streaming over HTTP (DASH) technology. The paper describes the different components of the testbed and provides examples of the KPI metrics that can be collected by using it. Different application contexts where the testbed can provide valuable benefits are also discussed.