prometheus: bundle the webui assets and build the new React UI

v2.14 introduced a new experimental React UI and changed how web assets
are bundled with Prometheus. A separate pre-build command is required to
generate an asset bundle that gets statically linked into the Prometheus
binary.

pkgs/servers/monitoring/prometheus/update-webui-deps.sh

@@ -0,0 +1,20 @@
+#!/usr/bin/env nix-shell
+#!nix-shell -I nixpkgs=../../../.. -i bash -p wget yarn2nix-moretea.yarn2nix
+
+# This script is based upon:
+# pkgs/applications/networking/instant-messengers/riot/update-riot-desktop.sh
+
+set -euo pipefail
+
+if [ "$#" -ne 1 ] || [[ "$1" == -* ]]; then
+        echo "Regenerates the Yarn dependency lock files for the prometheus package."
+        echo "Usage: $0 <git release tag>"
+        exit 1
+fi
+
+PROM_WEB_SRC="https://raw.githubusercontent.com/prometheus/prometheus/$1"
+
+wget "$PROM_WEB_SRC/web/ui/react-app/package.json" -O webui-package.json
+wget "$PROM_WEB_SRC/web/ui/react-app/yarn.lock" -O yarn.lock
+yarn2nix --lockfile=yarn.lock > webui-yarndeps.nix
+rm yarn.lock