Add prometheus export metrics functional testing

Without this, we can't know if the exposed prometheus metrics
behave properly.

This is a problem, as the only way we can evaluate the success
(right now), is a compilation success or failure from kured.
While this is a good start, it doesn't translate to what we
claim to offer: A boolean showing if a reboot is required.

This fixes it by creating a new github action workflow testing
if the float64 gauge is properly showing 0 for no reboot, 1 for reboot.
This is done by exposing the metrics endpoint through a node port.
A helm chart change was required to have the ability to expose
the service on a node port. We connect to the kind node through
docker in the `tests/test-metrics.sh`, where we curl the nodeport,
extract the only relevant metric, and compare it to the expected result.

tests/kind/test-metrics.sh

@@ -0,0 +1,19 @@
+#!/usr/bin/env bash
+
+expected="$1"
+if [[ "$expected" != "0" && "$expected" != "1" ]]; then
+    echo "You should give an argument to this script, the gauge value (0 or 1)"
+    exit 1
+fi
+
+HOST="${HOST:-localhost}"
+PORT="${PORT:-30000}"
+NODENAME="${NODENAME-chart-testing-control-plane}"
+
+reboot_required=$(docker exec "$NODENAME" curl "http://$HOST:$PORT/metrics" | awk '/^kured_reboot_required/{print $2}')
+if [[ "$reboot_required" == "$expected" ]]; then
+    echo "Test success"
+else
+    echo "Test failed"
+    exit 1
+fi