matmarex, catrope: Backport for [[gerrit:1253623|Fix client credentials access tokens (T417278 T419921)]], [[gerrit:1253625|Enable $wgTrackMediaRequestProvenance on testwikis and beta cluster (T414338)]], [[gerrit:1253626|Configure $wgApiClientErrorSampleRate (T418957)]] synced to the testservers (see https://wikitech.wikimedia.org/wiki/Mwdebug). Changes can now be verified there.