Add environment variable control for most command line options.

Adds the following environment variables for overriding defaults:
* `PG_EXPORTER_WEB_LISTEN_ADDRESS`
* `PG_EXPORTER_WEB_TELEMETRY_PATH`
* `PG_EXPORTER_EXTEND_QUERY_PATH`

Closes #150.

Author: wrouesnel

Diff for: postgres_exporter.go

@@ -32,9 +32,9 @@ import (
 var Version = "0.0.1"
 
 var (
-	listenAddress = kingpin.Flag("web.listen-address", "Address to listen on for web interface and telemetry.").Default(":9187").String()
-	metricPath    = kingpin.Flag("web.telemetry-path", "Path under which to expose metrics.").Default("/metrics").String()
-	queriesPath   = kingpin.Flag("extend.query-path", "Path to custom queries to run.").Default("").String()
+	listenAddress = kingpin.Flag("web.listen-address", "Address to listen on for web interface and telemetry.").Default(":9187").OverrideDefaultFromEnvar("PG_EXPORTER_WEB_LISTEN_ADDRESS").String()
+	metricPath    = kingpin.Flag("web.telemetry-path", "Path under which to expose metrics.").Default("/metrics").OverrideDefaultFromEnvar("PG_EXPORTER_WEB_TELEMETRY_PATH").String()
+	queriesPath   = kingpin.Flag("extend.query-path", "Path to custom queries to run.").Default("").OverrideDefaultFromEnvar("PG_EXPORTER_EXTEND_QUERY_PATH").String()
 	onlyDumpMaps  = kingpin.Flag("dumpmaps", "Do not run, simply dump the maps.").Bool()
 )
 
@@ -1108,8 +1108,8 @@ func main() {
 
 	http.Handle(*metricPath, promhttp.Handler())
 	http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
-    	w.Header().Set("Content-Type", "Content-Type:text/plain; charset=UTF-8") // nolint: errcheck
-		w.Write(landingPage) // nolint: errcheck
+		w.Header().Set("Content-Type", "Content-Type:text/plain; charset=UTF-8") // nolint: errcheck
+		w.Write(landingPage)                                                     // nolint: errcheck
 	})
 
 	log.Infof("Starting Server: %s", *listenAddress)