Cloud Computing represents a new trend in the development and use of software. The use of services hosted in the cloud by employing the Software as a Service (SaaS) model is currently being adopted by many organizations. Services are typically accompanied by a Service Level Agreement (SLA), which defines the quality terms that a provider offers to its customers. Many monitoring tools with which to report compliance with the SLA have been proposed. However, they have some limitations when it is necessary to make changes to monitoring requirements or owing to the complexity involved in capturing low-level raw data from services at runtime. In this paper, we propose a platform-independent monitoring middleware for cloud services, which supports the monitoring of SLA compliance and provides a report containing SLA violations that may help stakeholders to make decisions regarding how to improve the quality of cloud services. Moreover, our middleware definition is based on the use of models@runtime, which allows the dynamic change of quality requirements and/or the dynamic selection of different metric operationalizations (i.e., calculation formulas) with which to measure the quality of services. In order to demonstrate the feasibility of our approach, we show the instantiation of the proposed middleware that can be used to monitor services when deployed on the Microsoft Azure© platform.