Add a native Kubernetes secret provider

Signed-off-by: Daniel Hrabovcak <thespiritxiii@gmail.com>

secrets/kubernetes/kubernetes.go

@@ -0,0 +1,90 @@
+// Copyright 2024 The Prometheus Authors
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package kubernetes
+
+import (
+	"fmt"
+
+	"github.com/prometheus/common/config"
+	"github.com/prometheus/common/version"
+	corev1 "k8s.io/api/core/v1"
+	"k8s.io/apimachinery/pkg/types"
+	"k8s.io/client-go/kubernetes"
+	"k8s.io/client-go/rest"
+	"k8s.io/client-go/tools/clientcmd"
+)
+
+// HTTP header.
+var userAgent = fmt.Sprintf("Prometheus/%s", version.Version)
+
+// SecretConfig is the configuration pointing to a specific Kubernetes Secret.
+type SecretConfig struct {
+	Namespace string `yaml:"namespace"`
+	Name      string `yaml:"name"`
+	Key       string `yaml:"key"`
+}
+
+func (c *SecretConfig) objectKey() types.NamespacedName {
+	return types.NamespacedName{
+		Namespace: c.Namespace,
+		Name:      c.Name,
+	}
+}
+
+func getValue(s *corev1.Secret, key string) (string, error) {
+	if value, ok := s.Data[key]; ok {
+		return string(value), nil
+	}
+	if value, ok := s.StringData[key]; ok {
+		return value, nil
+	}
+	return "", fmt.Errorf("secret %s/%s does not contain key: %s", s.Namespace, s.Name, key)
+}
+
+// ClientConfig configures access to the Kubernetes API server.
+type ClientConfig struct {
+	APIServer  config.URL `yaml:"api_server,omitempty"`
+	KubeConfig string     `yaml:"kubeconfig_file,omitempty"`
+}
+
+func (c *ClientConfig) client() (kubernetes.Interface, error) {
+	var restClient *rest.Config
+	switch {
+	case c.KubeConfig != "":
+		var err error
+		restClient, err = clientcmd.BuildConfigFromFlags("", c.KubeConfig)
+		if err != nil {
+			return nil, err
+		}
+	case c.APIServer.URL == nil:
+		var err error
+		restClient, err = rest.InClusterConfig()
+		if err != nil {
+			return nil, err
+		}
+	}
+
+	restClient.UserAgent = userAgent
+	restClient.ContentType = "application/vnd.kubernetes.protobuf"
+
+	return kubernetes.NewForConfig(restClient)
+}
+
+func errNotFound(namespace, name string) error {
+	return fmt.Errorf("secret %s/%s not found or forbidden", namespace, name)
+}
+
+func errKeyNotFound(namespace, name, key string) error {
+	return fmt.Errorf("secret %s/%s does not contain key: %s", namespace, name, key)
+}