metrics: render Prometheus exposition for /metrics

Fill in the body that GET /metrics returns when format=prometheus.
The exposed series mirror what `show stat` prints over the CLI today:

  * accel_ppp_build_info{version="..."} 1
  * accel_ppp_uptime_seconds
  * accel_ppp_cpu_percent
  * accel_ppp_memory_{rss,virt}_bytes (read from /proc/<pid>/statm)
  * accel_ppp_core_mempool_{allocated,available}_bytes
  * accel_ppp_core_threads{,_active}
  * accel_ppp_core_contexts{,_sleeping,_pending}
  * accel_ppp_core_md_handlers{,_pending}
  * accel_ppp_core_timers{,_pending}
  * accel_ppp_sessions{state="starting|active|finishing"}
  * accel_ppp_protocol_sessions{protocol=...,state=...} for every
    protocol module that is currently loaded — pppoe, l2tp, pptp,
    sstp, ipoe — gated by triton_module_loaded() so we never call a
    stat helper from a module that wasn't loaded.

A small growing strbuf helper coalesces the rendering into a single
buffer that is passed to send_response() in one shot. Per-protocol
forward declarations rely on the existing RTLD_LAZY|RTLD_GLOBAL load
behaviour used by net-snmp, so the metrics .so does not need to link
against pppoe.so, l2tp.so, etc.

json format remains a stubbed `{}` body — the next commit replaces
that with a real renderer.

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

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