Filter out Bandit HTTP/0.9 errors from port scanners

Added log filter to suppress noisy errors from:
- HTTP/0.9 requests (obsolete protocol from 1991)
- Invalid HTTP version errors from port scanners and bots
- Malformed HTTP requests from automated tools

These errors are expected on public-facing servers and don't indicate
application problems. The filter uses Elixir's logger handler filters
to suppress them at the logging layer.

Resolves sporadic 'Bandit.HTTPError: Invalid HTTP version: {0, 9}' errors
in production logs.

config/config.exs

@@ -22,6 +22,14 @@ config :logger, :default_formatter,
   format: "$time $metadata[$level] $message\n",
   metadata: [:request_id]
 
+# Filter out noisy errors from port scanners and bots
+config :logger, :default_handler,
+  filters: [
+    # Suppress HTTP/0.9 and other invalid protocol errors from Bandit
+    # These are typically from port scanners and automated bots
+    bandit_invalid_http: {&Towerops.LogFilter.filter_bandit_errors/2, []}
+  ]
+
 # Register protobuf MIME type for agent API
 config :mime, :types, %{
   "application/x-protobuf" => ["protobuf"]

lib/towerops/log_filter.ex

@@ -0,0 +1,48 @@
+defmodule Towerops.LogFilter do
+  @moduledoc """
+  Filters out noisy log messages from external sources like port scanners and bots.
+  """
+
+  require Logger
+
+  @doc """
+  Filters out Bandit HTTP errors from invalid requests.
+
+  This suppresses errors from:
+  - HTTP/0.9 requests (very old protocol, typically port scanners)
+  - Malformed HTTP requests from bots
+  - Invalid protocol versions
+
+  These errors are expected when running a public-facing server and don't
+  indicate application problems.
+  """
+  def filter_bandit_errors(log_event, _opts) do
+    case log_event do
+      # Filter Bandit.HTTPError with Invalid HTTP version
+      %{level: :error, msg: {:string, msg}} when is_binary(msg) ->
+        filter_string_message(msg)
+
+      # Filter other Bandit connection errors
+      %{level: :error, msg: {:report, %{label: {:error_logger, :error_report}, report: report}}} ->
+        filter_error_report(report)
+
+      # Don't filter anything else
+      _ ->
+        :ignore
+    end
+  end
+
+  defp filter_string_message(msg) do
+    if bandit_invalid_http_error?(msg), do: :stop, else: :ignore
+  end
+
+  defp filter_error_report(message: msg) when is_binary(msg) do
+    if String.contains?(msg, "Bandit.HTTPError"), do: :stop, else: :ignore
+  end
+
+  defp filter_error_report(_), do: :ignore
+
+  defp bandit_invalid_http_error?(msg) do
+    String.contains?(msg, "Bandit.HTTPError") and String.contains?(msg, "Invalid HTTP version")
+  end
+end