metrics: back off accept loop on persistent errors

Previously the only path out of serv_read()'s accept loop was an
EAGAIN/EWOULDBLOCK return; every other failure logged once and fell
back into `continue`. With a level-readable listening fd, that means
EMFILE/ENFILE/ENOBUFS/ENOMEM pin the worker thread spinning on
accept() and saturate the log.

Detect that class of error and pause the listener: disable
MD_MODE_READ on serv_hnd, arm a one-shot triton timer for one
second, and on expiry re-enable the handler. EINTR and ECONNABORTED
are kept as transient retries — those are normal and short-lived.

Verified by running the daemon under `prlimit --nofile=24` and
opening enough slow connections to exhaust the limit. The first
accept failure logs

  metrics: accept failed: Too many open files; backing off 1s

then the daemon idles at 0% CPU instead of spinning; once fds free
up it resumes accepting.

Signed-off-by: Denys Fedoryshchenko <denys.f@collabora.com>

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