metrics: scaffold module with config parsing

Add a new extra module `metrics` that will eventually expose statistics
over HTTP. This first commit only:

  * creates accel-pppd/extra/metrics.c with an init() that parses the
    [metrics] section options `format` (prometheus|json) and
    `address` (host:port);
  * wires the new shared library into the extras CMakeLists.

No listener, no metrics rendering yet — those land in follow-up
commits. With this commit alone, loading the `metrics` module just
logs the configured listen address and format and is otherwise inert.

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

0 files changed, 0 insertions, 0 deletions