Use prometheus configfile relabel objects

[alfredolopezzz]

Context

We allow users to supply a cnfigmap with relabelling configuration in
YAML. We parse that into an object provided by Prometheus-Operator,
as we are adding parts to an object defined by the operator.

Unfortunately, the operator object is only tagged for JSON marshaling
and results in a differnt syntax than the one documented for
Prometheus config files.

Goals

Allow the same syntax for the relabeling configmap as Prometheus
uses in its config files (what we document publicly)

[alfredolopezzz]

previous approach (wip):

diff --git a/pkg/controllers/podmonitor_controller.go b/pkg/controllers/podmonitor_controller.go
index 7506c2b..9895a99 100644
--- a/pkg/controllers/podmonitor_controller.go
+++ b/pkg/controllers/podmonitor_controller.go
@@ -16,6 +16,7 @@ import (
 	promcommonconfig "github.com/prometheus/common/config"
 	promconfig "github.com/prometheus/prometheus/config"
 	"github.com/prometheus/prometheus/discovery/kubernetes"
+	"github.com/prometheus/prometheus/pkg/relabel"
 	"gopkg.in/yaml.v3"
 	autoscaling "k8s.io/api/autoscaling/v1"
 	corev1 "k8s.io/api/core/v1"
@@ -477,8 +478,8 @@ func (r *PodMonitorReconciler) getTenantCustomRelabelConfigs(namespace string, p
 		return existingTenantCustomRelabelConfigs
 	}
 
-	var configs []monitoringv1.RelabelConfig
-	err = yaml.Unmarshal([]byte(relabelConfigs), &configs)
+	var userConfigs []relabel.Config
+	err = yaml.Unmarshal([]byte(relabelConfigs), &userConfigs)
 	if err != nil {
 		r.Log.Error(err, "Failed to unmarshal relabel-configs")
 
@@ -492,11 +493,39 @@ func (r *PodMonitorReconciler) getTenantCustomRelabelConfigs(namespace string, p
 
 	r.Log.Info(fmt.Sprintf("Successfully fetched Custom RelabelConfig ConfigMap Name for %s", namespace))
 
+	var configs []monitoringv1.RelabelConfig
+	for _, config := range userConfigs {
+		configs = append(
+			configs,
+			convertPrometheusRelabelConfigToOperatorsRelabelConfig(config),
+		)
+	}
 	configs = append(configs, defaultPrometheusConfig)
 
 	return configs
 }
 
+func convertPrometheusRelabelConfigToOperatorsRelabelConfig(config relabel.Config) monitoringv1.RelabelConfig {
+	var sourceLabels []string
+	for _, label := range config.SourceLabels {
+		sourceLabels = append(sourceLabels, string(label))
+	}
+
+	regex := config.Regex.String()
+	regex = strings.TrimPrefix(regex, "^(?:")
+	regex = strings.TrimSuffix(regex, ")$")
+
+	return monitoringv1.RelabelConfig{
+		SourceLabels: sourceLabels,
+		Separator:    config.Separator,
+		TargetLabel:  config.TargetLabel,
+		Regex:        regex,
+		Modulus:      config.Modulus,
+		Replacement:  config.Replacement,
+		Action:       string(config.Action),
+	}
+}
+
 func (r *PodMonitorReconciler) updatePrometheusObjectDef(monitor monitoringv1.PodMonitor, prometheus *monitoringv1.Prometheus, grafanaStackTenantMapping map[string]*grafanacloud.Stack, nodeSelectorTarget string) error {
 	replicas := int32(1)
 	shards := int32(1)
diff --git a/pkg/controllers/podmonitor_controller_test.go b/pkg/controllers/podmonitor_controller_test.go
index bd9e588..6186271 100644
--- a/pkg/controllers/podmonitor_controller_test.go
+++ b/pkg/controllers/podmonitor_controller_test.go
@@ -608,12 +608,16 @@ func TestPodMonitorWithCustomRelabelConfigs(t *testing.T) {
 		expectedPrometheus.ResourceVersion = prometheuses.Items[0].ResourceVersion
 		expectedPrometheus.Spec.RemoteWrite[0].WriteRelabelConfigs = []monitoringv1.RelabelConfig{
 			{
-				Action: "labeldrop",
-				Regex:  "^prometheus$",
+				Action:      "labeldrop",
+				Regex:       "^prometheus$",
+				Replacement: "$1",
+				Separator:   ";",
 			},
 			{
-				Action: "labeldrop",
-				Regex:  "^node_exporter$",
+				Action:      "labeldrop",
+				Regex:       "^node_exporter$",
+				Replacement: "$1",
+				Separator:   ";",
 			},
 			{
 				Action:       "drop",