Adjust threshold for SpaceFillingUp warning alert

Reduce threshold of NodeFilesystemSpaceFillingUp warning alert to 20% space available, instead of 40% (default).

This will align the threshold according to default kubelet GC values
below[1],

"imageMinimumGCAge": "2m0s",
"imageGCHighThresholdPercent": 85,
"imageGCLowThresholdPercent": 80,

[1] https://kubernetes.io/docs/reference/config-api/kubelet-config.v1beta1/

Signed-off-by: Arunprasad Rajkumar <arajkuma@redhat.com>

jsonnet/kube-prometheus/components/node-exporter.libsonnet

@@ -28,6 +28,7 @@ local defaults = {
     ruleLabels: {},
     _config: {
       nodeExporterSelector: 'job="' + defaults.name + '"',
+      fsSpaceFillingUpWarningThreshold: 20,
       fsSpaceFillingUpCriticalThreshold: 15,
       diskDeviceSelector: 'device=~"mmcblk.p.+|nvme.+|rbd.+|sd.+|vd.+|xvd.+|dm-.+|dasd.+"',
       runbookURLPattern: 'https://runbooks.prometheus-operator.dev/runbooks/node/%s',