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
examplethe total number of requests submiited. Ideally these data should be kept “per-user” and of course with the
properlocking 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
fieldsin the query planning structures and callbacks but these do not seem to be relevant to my requirements. Another
possiblityseems to be to use shared memory (https://www.postgresql.org/docs/12/xfunc-c.html#id-1.8.3.13.14) but it is
evenless clear how to do it. 

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