pkg/prometheus: make replicas label configurable

This change enables existing Thanos deployments which adopted any label
name other than `prometheus_replica` to migrate to Prometheus Operator
without losing of Thanos deduplication features.

Defaults to `prometheus_replica` for backward compatibility.

Documentation/api.md

@@ -274,6 +274,7 @@ PrometheusSpec is a specification of the desired behavior of the Prometheus clus
 | baseImage | Base image to use for a Prometheus deployment. | string | false |
 | imagePullSecrets | An optional list of references to secrets in the same namespace to use for pulling prometheus and alertmanager images from registries see http://kubernetes.io/docs/user-guide/images#specifying-imagepullsecrets-on-a-pod | [][v1.LocalObjectReference](https://kubernetes.io/docs/reference/generated/kubernetes-api/v1.11/#localobjectreference-v1-core) | false |
 | replicas | Number of instances to deploy for a Prometheus deployment. | *int32 | false |
+| replicaExternalLabelName | Name of Prometheus external label used to denote replica name. Defaults to the value of `prometheus_replica`. | string | false |
 | retention | Time duration Prometheus shall retain data for. Default is '24h', and must match the regular expression `[0-9]+(ms\|s\|m\|h\|d\|w\|y)` (milliseconds seconds minutes hours days weeks years). | string | false |
 | logLevel | Log level for Prometheus to be configured with. | string | false |
 | logFormat | Log format for Prometheus to be configured with. | string | false |

example/prometheus-operator-crd/prometheus.crd.yaml

@@ -2147,6 +2147,10 @@ spec:
                 required:
                 - url
               type: array
+            replicaExternalLabelName:
+              description: Name of Prometheus external label used to denote replica
+                name. Defaults to the value of `prometheus_replica`.
+              type: string
             replicas:
               description: Number of instances to deploy for a Prometheus deployment.
               format: int32

pkg/apis/monitoring/v1/openapi_generated.go

@@ -1298,6 +1298,13 @@ func schema_pkg_apis_monitoring_v1_PrometheusSpec(ref common.ReferenceCallback)
 							Format:      "int32",
 						},
 					},
+					"replicaExternalLabelName": {
+						SchemaProps: spec.SchemaProps{
+							Description: "Name of Prometheus external label used to denote replica name. Defaults to the value of `prometheus_replica`.",
+							Type:        []string{"string"},
+							Format:      "",
+						},
+					},
 					"retention": {
 						SchemaProps: spec.SchemaProps{
 							Description: "Time duration Prometheus shall retain data for. Default is '24h', and must match the regular expression `[0-9]+(ms|s|m|h|d|w|y)` (milliseconds seconds minutes hours days weeks years).",

pkg/apis/monitoring/v1/types.go

@@ -109,6 +109,9 @@ type PrometheusSpec struct {
 	ImagePullSecrets []v1.LocalObjectReference `json:"imagePullSecrets,omitempty"`
 	// Number of instances to deploy for a Prometheus deployment.
 	Replicas *int32 `json:"replicas,omitempty"`
+	// Name of Prometheus external label used to denote replica name.
+	// Defaults to the value of `prometheus_replica`.
+	ReplicaExternalLabelName string `json:"replicaExternalLabelName,omitempty"`
 	// Time duration Prometheus shall retain data for. Default is '24h',
 	// and must match the regular expression `[0-9]+(ms|s|m|h|d|w|y)` (milliseconds seconds minutes hours days weeks years).
 	Retention string `json:"retention,omitempty"`

pkg/prometheus/promcfg.go

@@ -91,7 +91,13 @@ func buildExternalLabels(p *v1.Prometheus) yaml.MapSlice {
 	m := map[string]string{}
 
 	m["prometheus"] = fmt.Sprintf("%s/%s", p.Namespace, p.Name)
-	m["prometheus_replica"] = "$(POD_NAME)"
+
+	replicaExternalLabelName := p.Spec.ReplicaExternalLabelName
+	if replicaExternalLabelName == "" {
+		replicaExternalLabelName = defaultReplicaExternalLabelName
+	}
+
+	m[replicaExternalLabelName] = "$(POD_NAME)"
 
 	for n, v := range p.Spec.ExternalLabels {
 		m[n] = v
@@ -195,11 +201,15 @@ func (cg *configGenerator) generateConfig(
 	var alertRelabelConfigs []yaml.MapSlice
 
 	// action 'labeldrop' is not supported <= v1.4.1
+	replicaExternalLabelName := p.Spec.ReplicaExternalLabelName
+	if replicaExternalLabelName == "" {
+		replicaExternalLabelName = defaultReplicaExternalLabelName
+	}
 	if version.GT(semver.MustParse("1.4.1")) {
-		// Drop 'prometheus_replica' label, to make alerts from two Prometheus replicas alike
+		// Drop replica label, to make alerts from multiple Prometheus replicas alike
 		alertRelabelConfigs = append(alertRelabelConfigs, yaml.MapSlice{
 			{Key: "action", Value: "labeldrop"},
-			{Key: "regex", Value: "prometheus_replica"},
+			{Key: "regex", Value: regexp.QuoteMeta(replicaExternalLabelName)},
 		})
 	}
 

pkg/prometheus/statefulset.go

@@ -33,19 +33,20 @@ import (
 )
 
 const (
-	governingServiceName     = "prometheus-operated"
-	DefaultPrometheusVersion = "v2.7.1"
-	DefaultThanosVersion     = "v0.2.1"
-	defaultRetention         = "24h"
-	storageDir               = "/prometheus"
-	confDir                  = "/etc/prometheus/config"
-	confOutDir               = "/etc/prometheus/config_out"
-	rulesDir                 = "/etc/prometheus/rules"
-	secretsDir               = "/etc/prometheus/secrets/"
-	configmapsDir            = "/etc/prometheus/configmaps/"
-	configFilename           = "prometheus.yaml.gz"
-	configEnvsubstFilename   = "prometheus.env.yaml"
-	sSetInputHashName        = "prometheus-operator-input-hash"
+	governingServiceName            = "prometheus-operated"
+	DefaultPrometheusVersion        = "v2.7.1"
+	DefaultThanosVersion            = "v0.2.1"
+	defaultRetention                = "24h"
+	defaultReplicaExternalLabelName = "prometheus_replica"
+	storageDir                      = "/prometheus"
+	confDir                         = "/etc/prometheus/config"
+	confOutDir                      = "/etc/prometheus/config_out"
+	rulesDir                        = "/etc/prometheus/rules"
+	secretsDir                      = "/etc/prometheus/secrets/"
+	configmapsDir                   = "/etc/prometheus/configmaps/"
+	configFilename                  = "prometheus.yaml.gz"
+	configEnvsubstFilename          = "prometheus.env.yaml"
+	sSetInputHashName               = "prometheus-operator-input-hash"
 )
 
 var (