metrics: add HTTP listener with /metrics endpoint

Stand up a minimal HTTP/1.1 server on top of triton's md handler, using
the same pattern as cli/tcp.c. The listener:

  * binds the address configured in [metrics]/address (IPv4 only for
    now);
  * accepts non-blocking connections, reads up to the first
    "\r\n\r\n" into a fixed-size buffer (2 KiB), then dispatches one
    request and closes the connection;
  * routes GET /metrics to a placeholder 200 response (body is empty
    until the metrics rendering lands in a later commit);
  * returns 404 for other paths, 405 for non-GET, 413 if the request
    headers do not fit, and 400 for an unparseable request line.

Content-Type is selected from the configured format (prometheus or
json) so the next commits can plug in real bodies without touching the
transport.

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

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