prometheus-knot-exporter: add patch to fix stats

This is a patch I filed against upstream[1] a while ago. As it isn't
merged yet and fixes configurations with all stats enabled in knot
(otherwise it'd crash when sending a request to `localhost:9433`), I
decided that it makes sense to add it to the package directly.

I extended the test to make sure that it only passes with this patch.

[1] https://github.com/ghedo/knot_exporter/pull/6

pkgs/servers/monitoring/prometheus/knot-exporter.nix

@@ -1,4 +1,4 @@
-{ stdenv, fetchFromGitHub, lib, python3, nixosTests }:
+{ stdenv, fetchFromGitHub, lib, python3, nixosTests, fetchpatch }:
 
 stdenv.mkDerivation rec {
   pname = "knot-exporter";
@@ -11,6 +11,15 @@ stdenv.mkDerivation rec {
     sha256 = "sha256-4au4lpaq3jcqC2JXdCcf8h+YN8Nmm4eE0kZwA+1rWlc=";
   };
 
+  patches = [
+    # Fixes a crash with all metrics enabled. See
+    # https://github.com/ghedo/knot_exporter/pull/6 for further context.
+    (fetchpatch {
+      url = "https://github.com/ghedo/knot_exporter/commit/2317476e080369450ae51a707ccd30d4b89d680f.patch";
+      sha256 = "sha256-yEPu8EE1V/draNx9DeMrPj+bMfJRxauweo33dITl4AA=";
+    })
+  ];
+
   dontBuild = true;
 
   nativeBuildInputs = [ python3.pkgs.wrapPython ];