Hi @Patrick_S -
So, this is a nice find. I reproduced your result that while the wt binary produces .gcda files, the mongod binary apparently does not. Thanks for taking the time to write it up and let us know.
Would you please open a SERVER ticket in the MongoDB JIRA at https://jira.mongodb.org/projects/SERVER describing your findings?
If you would like to continue investigating, other interesting binaries to try out include mongo, mongos, and unit tests like base_test. Please feel free to include any results you may generate for those targets in your ticket and link back to this discussion.
Thanks,
Andrew