Fix unable to open StatsD socket. reason: :emfile

[marcos-sandim]

If StatsD is not available and the packet transmission fails, Peep.StatsD.send_packets/3 removes the socket from the state but does not close it.

This leads to a situation where we try to open a new socket every few seconds and don't close any of them and end up hitting the maximum file descriptors limit.

This change simply closes the socket, freeing up the file descriptor.

Error:

[error] 2025-09-12 17:15:57.855 unable to open StatsD socket. reason: :emfile
        {line=150 pid=<0.440.0> file=lib/peep/statsd.ex domain=elixir application=peep mfa=Peep.Statsd.try_to_open_socket/1 }

[rkallos]

Thanks for the PR!

I don't see much harm in merging your PR, but I was unable to write a test showcasing the bug that your change claims to fix.

To the best of my knowledge, when a Peep process terminates, the Peep process's UDP socket also gets closed, because it was the UDP socket's owner/controlling process.

This test reliably passes on my machine:

test "UDP socket is closed when Peep process terminates" do
  name = :"#{__MODULE__}_udp_socket_test"

  counter = Metrics.counter("#{name}.counter", event_name: [name, :counter])

  {:ok, options} =
    [
      name: name,
      metrics: [counter],
      statsd: [
        host: "localhost",
        port: 8125,
        flush_interval_ms: 1000
      ]
    ]
    |> Peep.Options.validate()

  {:ok, pid} = GenServer.start(Peep, options, name: options.name)

  state = :sys.get_state(pid)

  assert state.statsd_state != nil
  assert state.statsd_state.socket != nil

  socket = state.statsd_state.socket

  assert match?({:ok, _}, :inet.port(socket))

  GenServer.stop(pid, :shutdown)

  assert match?({:error, :einval}, :inet.port(socket))
end