prevent prometheus sts update loop

Changes in kubernetes v1.17 cause an endless update loop due
to validation errors cause by setting the 'apiVersion' and
'kind' fields in the StatefulSet spec.  These two fields are
set to empty strings by the Kube API server, and v1.17 added
validation that does not allow these fields to be modified.

See prometheus-operator#2950

pkg/prometheus/operator.go

@@ -1145,6 +1145,7 @@ func (c *Operator) sync(key string) error {
 	if err != nil {
 		return errors.Wrap(err, "making statefulset failed")
 	}
+	sanitizeSTS(sset)
 
 	if !exists {
 		level.Debug(c.logger).Log("msg", "no current Prometheus statefulset found")
@@ -1183,6 +1184,15 @@ func (c *Operator) sync(key string) error {
 	return nil
 }
 
+// sanitizeSTS removes values for APIVersion and Kind from the VolumeClaimTemplates.
+// This prevents update failures due to these fields changing when applied.
+func sanitizeSTS(sts *appsv1.StatefulSet) {
+	for i := range sts.Spec.VolumeClaimTemplates {
+		sts.Spec.VolumeClaimTemplates[i].APIVersion = ""
+		sts.Spec.VolumeClaimTemplates[i].Kind = ""
+	}
+}
+
 //checkPrometheusSpecDeprecation checks for deprecated fields in the prometheus spec and logs a warning if applicable
 func checkPrometheusSpecDeprecation(key string, p *monitoringv1.Prometheus, logger log.Logger) {
 	deprecationWarningf := "prometheus key=%v, field %v is deprecated, '%v' field should be used instead"