The challenge of how to make networking technology "greener", i.e. more environmentally sustainable, has recently been gaining a lot of attention. One key to potential solutions concerns the ability to have visibility into metrics that can be used to assess and optimize networks’ environmental footprint. While past attention has focused on individual device metrics, such as power consumption, certain use cases would also benefit from more holistic metrics that apply to networking paths.This paper describes an approach for instrumenting a network with metrics that indicate the environmental efficiency of paths, obtained by aggregating a set of configurable device efficiency metrics along networking paths. The aggregation occurs using a new extension to In-situ Operations Administration and Maintenance (IOAM) as a packet traverses hops in the data plane, without depending on controllers or needing to collect individual data records that might lead to Maximum Transmission Unit (MTU) issues associated with other versions of IOAM. Particular emphasis is given to the efficiency of aggregation operations to minimize performance impact. A proof of concept has been developed using P4.