chore: (pre-release) add health check command

cmd/postgres_exporter/main.go

@@ -14,7 +14,9 @@
 package main
 
 import (
+	"errors"
 	"fmt"
+	"net"
 	"net/http"
 	"os"
 	"strings"
@@ -46,6 +48,7 @@ var (
 	autoDiscoverDatabases  = kingpin.Flag("auto-discover-databases", "Whether to discover the databases on a server dynamically. (DEPRECATED)").Default("false").Envar("PG_EXPORTER_AUTO_DISCOVER_DATABASES").Bool()
 	queriesPath            = kingpin.Flag("extend.query-path", "Path to custom queries to run. (DEPRECATED)").Default("").Envar("PG_EXPORTER_EXTEND_QUERY_PATH").String()
 	onlyDumpMaps           = kingpin.Flag("dumpmaps", "Do not run, simply dump the maps.").Bool()
+	health                 = kingpin.Flag("health", "Do not run, just return if up and running.").Bool()
 	constantLabelsList     = kingpin.Flag("constantLabels", "A list of label=value separated by comma(,). (DEPRECATED)").Default("").Envar("PG_EXPORTER_CONSTANT_LABELS").String()
 	excludeDatabases       = kingpin.Flag("exclude-databases", "A list of databases to remove when autoDiscoverDatabases is enabled (DEPRECATED)").Default("").Envar("PG_EXPORTER_EXCLUDE_DATABASES").String()
 	includeDatabases       = kingpin.Flag("include-databases", "A list of databases to include when autoDiscoverDatabases is enabled (DEPRECATED)").Default("").Envar("PG_EXPORTER_INCLUDE_DATABASES").String()
@@ -81,6 +84,17 @@ func main() {
 		return
 	}
 
+	if *health {
+		healthy, err := runHealthCheck(webConfig)
+		if err != nil {
+			level.Error(logger).Log("msg", "Error running helath check", "err", err)
+		}
+		if healthy {
+			os.Exit(0)
+		}
+		os.Exit(1)
+	}
+
 	if err := c.ReloadConfig(*configFile, logger); err != nil {
 		// This is not fatal, but it means that auth must be provided for every dsn.
 		level.Warn(logger).Log("msg", "Error loading config", "err", err)
@@ -174,3 +188,23 @@ func main() {
 		os.Exit(1)
 	}
 }
+
+func runHealthCheck(webConfig *web.FlagConfig) (bool, error) {
+	if len(*webConfig.WebListenAddresses) == 0 {
+		return false, errors.New("no listen addresses to run the request to")
+	}
+	addr := (*webConfig.WebListenAddresses)[0]
+	url := fmt.Sprintf("http://%s/", &addr)
+	req, err := http.NewRequest("GET", url, nil)
+    if err != nil {
+        return false, err
+    }
+
+    resp, err := http.DefaultClient.Do(req)
+    if err != nil {
+        return false, err
+    }
+
+    defer resp.Body.Close()
+    return resp.StatusCode == 200, nil
+}