Keeping state in a foreign data wrapper

Hi,

I am trying to implement a FDW in Postgres for accessing a web api and I would like to keep information like for example the total number of requests submiited. Ideally these data should be kept “per-user” and of course with the proper locking to eliminate race conditions. So the question I have is how to design such a global (but per user and database) state, using the C FDW API of Postgres. I have gone through the API and for example I see various fdw_private fields in the query planning structures and callbacks but these do not seem to be relevant to my requirements. Another possiblity seems to be to use shared memory (https://www.postgresql.org/docs/12/xfunc-c.html#id-1.8.3.13.14) but it is even less clear how to do it.

So is there any simple way to do implement such shared thread/process safe state?
Thank you
Stelios