Add initial support for YAML configuration files

Signed-off-by: Ben Reedy <breed808@breed808.com>

exporter.go

@@ -18,6 +18,7 @@ import (
 
 	"github.com/StackExchange/wmi"
 	"github.com/prometheus-community/windows_exporter/collector"
+	"github.com/prometheus-community/windows_exporter/config"
 	"github.com/prometheus/client_golang/prometheus"
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 	"github.com/prometheus/common/log"
@@ -269,7 +270,11 @@ func initWbem() {
 
 func main() {
 	var (
-		app           = kingpin.New("windows_exporter", "")
+		app        = kingpin.New("windows_exporter", "")
+		configFile = app.Flag(
+			"config.file",
+			"YAML configuration file to use. Values set in this file will override flags.",
+		).Default(os.ExpandEnv("$ProgramFiles\\windows_exporter\\config.yml")).String()
 		listenAddress = app.Flag(
 			"telemetry.addr",
 			"host:port for exporter.",
@@ -299,7 +304,26 @@ func main() {
 	log.AddFlags(app)
 	app.Version(version.Print("windows_exporter"))
 	app.HelpFlag.Short('h')
-	app.Parse(os.Args[1:])
+
+	// Load values from configuration file(s). Executable flags must first be parsed, in order
+	// to load the specified file(s).
+	_, err := app.Parse(os.Args[1:])
+	if err != nil {
+		log.Fatalf("%v\n", err)
+	}
+	resolver, err := config.NewResolver(*configFile)
+	if err != nil {
+		log.Fatalf("could not load config file: %v\n", err)
+	}
+	err = resolver.Bind(app, os.Args[1:])
+	if err != nil {
+		log.Fatalf("%v\n", err)
+	}
+	// Parse flags once more to include those discovered in configuration file(s).
+	_, err = app.Parse(os.Args[1:])
+	if err != nil {
+		log.Fatalf("%v\n", err)
+	}
 
 	if *printCollectors {
 		collectors := collector.Available()