Try creating a package to retrieve megacli2prom

nixos/pkgs/megacli2prom/default.nix

+{ pkgs ? import <nixpkgs> {} }:
+
+pkgs.stdenv.mkDerivation {
+  name = "megacli2prom";
+  buildInputs = [ pkgs.megacli ];
+  src = pkgs.fetchFromGitHub {
+    owner = "PrivateStorageio";
+    repo = "megacli2prom";
+    rev = "43a5ceeed4219f013ac57a0ec3e76c4c9d2f4d93";
+    # sha256 = "0000000000000000000000000000000000000000000000000000";
+    sha256 = "1b8j3sg77j7bqg8529zyxfcha2scqdn9b8xvwqv5drcj4r121klc";
+  };
+  # substituteAllInPlace
+  installPhase = ''
+    mkdir -p $out/bin
+    cp ./megacli2prom.py $out/bin/megacli2prom
+    chmod +x $out/bin/megacli2prom
+  '';
+}
+