Side request access logging

(apologies if this was filed earlier, but I can't find it)

Many of existing extensions issue side requests (e.g. fetching JWKs, ext_*, telemetry export). When those requests fail, it's difficult to find out the root cause, since we only observe the effects on the extension behavior (e.g. failure to authenticate JWT, missing telemetry). It would be good to have a direct observation of the failed side requests, via a log entry which could provide the exact failure. We could also make this configurable independently of the extension, to allow dynamic troubleshooting (e.g. runtime override, admin endpoint, or regular xDS separate from filter config).