Man pages matching name pmdasetfetchcallback in section 3:

pmdaSetFetchCallBack.3.en	debian 10 / libpcp-pmda3-dev
pmdaSetFetchCallBack.3.en	debian 11 / libpcp-pmda3-dev
pmdaSetFetchCallBack.3.en	debian 12 / libpcp-pmda3-dev