Add config module

The config module supports adding configuration to the exporter via a config file. This supports adding authentication details in a config file so that /probe requests can specify authentication for endpoints

Signed-off-by: Joe Adams <github@joeadams.io>

config/config.go

@@ -0,0 +1,126 @@
+// Copyright 2022 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 config
+
+import (
+	"fmt"
+	"net/url"
+	"os"
+	"strings"
+	"sync"
+
+	"github.com/go-kit/log"
+	"github.com/prometheus/client_golang/prometheus"
+	"gopkg.in/yaml.v3"
+)
+
+var (
+	configReloadSuccess = prometheus.NewGauge(prometheus.GaugeOpts{
+		Namespace: "postgres_exporter",
+		Name:      "config_last_reload_successful",
+		Help:      "Postgres exporter config loaded successfully.",
+	})
+
+	configReloadSeconds = prometheus.NewGauge(prometheus.GaugeOpts{
+		Namespace: "postgres_exporter",
+		Name:      "config_last_reload_success_timestamp_seconds",
+		Help:      "Timestamp of the last successful configuration reload.",
+	})
+)
+
+func init() {
+	prometheus.MustRegister(configReloadSuccess)
+	prometheus.MustRegister(configReloadSeconds)
+}
+
+type Config struct {
+	AuthModules map[string]AuthModule `yaml:"auth_modules"`
+}
+
+type AuthModule struct {
+	Type     string   `yaml:"type"`
+	UserPass UserPass `yaml:"userpass,omitempty"`
+	// Add alternative auth modules here
+	Options map[string]string `yaml:"options"`
+}
+
+type UserPass struct {
+	Username string `yaml:"username"`
+	Password string `yaml:"password"`
+}
+
+type ConfigHandler struct {
+	sync.RWMutex
+	Config *Config
+}
+
+func (ch *ConfigHandler) GetConfig() *Config {
+	ch.RLock()
+	defer ch.RUnlock()
+	return ch.Config
+}
+
+func (ch *ConfigHandler) ReloadConfig(f string, logger log.Logger) error {
+	config := &Config{}
+	var err error
+	defer func() {
+		if err != nil {
+			configReloadSuccess.Set(0)
+		} else {
+			configReloadSuccess.Set(1)
+			configReloadSeconds.SetToCurrentTime()
+		}
+	}()
+
+	yamlReader, err := os.Open(f)
+	if err != nil {
+		return fmt.Errorf("Error opening config file %q: %s", f, err)
+	}
+	defer yamlReader.Close()
+	decoder := yaml.NewDecoder(yamlReader)
+	decoder.KnownFields(true)
+
+	if err = decoder.Decode(config); err != nil {
+		return fmt.Errorf("Error parsing config file %q: %s", f, err)
+	}
+
+	ch.Lock()
+	ch.Config = config
+	ch.Unlock()
+	return nil
+}
+
+func (m AuthModule) ConfigureTarget(target string) (string, error) {
+	// ip:port urls do not parse properly and that is the typical way users interact with postgres
+	t := fmt.Sprintf("exporter://%s", target)
+	u, err := url.Parse(t)
+	if err != nil {
+		return "", err
+	}
+
+	if m.Type == "userpass" {
+		u.User = url.UserPassword(m.UserPass.Username, m.UserPass.Password)
+	}
+
+	query := u.Query()
+	for k, v := range m.Options {
+		query.Set(k, v)
+	}
+	u.RawQuery = query.Encode()
+
+	parsed := u.String()
+	trim := strings.TrimPrefix(parsed, "exporter://")
+
+	return trim, nil
+}