libcoap | Examples that use the libCoAP library | Runtime Evironment library

The ESP-IDF build system is built on top of CMake. This means you can use all the standard features of CMake in your files. However, the the ESP-IDF system predefines many functions, and makes many assumptions about the layout of your project, supposedly to make things "easier". Instead of reading CMake documentation, start by reading and understanding the ESP-IDF build system documentation:

https://docs.espressif.com/projects/esp-idf/en/latest/esp32/api-guides/build-system.html

It looks to me like there is a particular layout expected for subcomponents, including the format of the CMakeLists.txt file. Specifically, move Metriful under a new directory called components, or add Metriful to EXTRA_COMPONENT_DIRS near the top of your root CMakeLists.txt

If Metriful is not written as an esp-idf component, this may not work. However, the document also describes how to link to "pure CMake" components, which will look something like this (at the end of your root CMakeLists.txt).