For years, PR was measured as a single entity from the time I lead a group of broadcasters to get Asesores, Inc., who did the survey for over 20 years before Arbitron arrived, to do a "Puerto Rico Consolidated" book. It always had regional breakouts for the different regions, but since the agencies did not buy single local stations much, that was just to appease the protesters.
Arbitron provides preconfigured "regions" which are a lot like embedded markets on the mainland. Subscribers can do regional reports but that is seldom done.
All the significant players in PR operate as networks... several stations in a virtual simulcast to cover the entire Island. So, for all practical purposes, all the major stations do cover the Island entirely.
The reason for the "whole Island" approach is that historically all newspapers were published in San Juan and distributed island-wide. Then in the 50's TV also originated from San Juan with only enough local content on the outlying licensed stations to meet FCC requirements. And by the late 70's and early 80's radio did the same (although networking for news blocks goes back 60 years or more).
Underlying this is the structure of Puerto Rican government and related entities which are all centralized. The individual municipalities (equivalent of counties and cities all rolled into one) don't have police departments, don't have school systems, don't collect property tax, etc. It all comes out of San Juan. So naturally all media of importance originates there.