Filter list of collectors enabled by default

Enabled by default collectors are chosen for Linux, which supports all
of the implemented collectors. But for other OSes (OS X, for example)
this list is not suitable, because they lack most of those collectors.

Because of that, it is not possible to run node_exporter with default
options on such OSes. Fix this by filtering list of enabled by default
collectors based on their availability for current platform.

Closes #149

Signed-off-by: Pavel Borzenkov <pavel.borzenkov@gmail.com>

node_exporter.go

@@ -41,8 +41,10 @@ var (
 	memProfile        = flag.String("debug.memprofile-file", "", "Write memory profile to this file upon receipt of SIGUSR1.")
 	listenAddress     = flag.String("web.listen-address", ":9100", "Address on which to expose metrics and web interface.")
 	metricsPath       = flag.String("web.telemetry-path", "/metrics", "Path under which to expose metrics.")
-	enabledCollectors = flag.String("collectors.enabled", "diskstats,filefd,filesystem,loadavg,mdadm,meminfo,netdev,netstat,sockstat,stat,textfile,time,uname", "Comma-separated list of collectors to use.")
-	printCollectors   = flag.Bool("collectors.print", false, "If true, print available collectors and exit.")
+	enabledCollectors = flag.String("collectors.enabled",
+		filterAvailableCollectors("diskstats,filefd,filesystem,loadavg,mdadm,meminfo,netdev,netstat,sockstat,stat,textfile,time,uname"),
+		"Comma-separated list of collectors to use.")
+	printCollectors = flag.Bool("collectors.print", false, "If true, print available collectors and exit.")
 
 	collectorLabelNames = []string{"collector", "result"}
 
@@ -81,6 +83,17 @@ func (n NodeCollector) Collect(ch chan<- prometheus.Metric) {
 	scrapeDurations.Collect(ch)
 }
 
+func filterAvailableCollectors(collectors string) string {
+	availableCollectors := make([]string, 0)
+	for _, c := range strings.Split(collectors, ",") {
+		_, ok := collector.Factories[c]
+		if ok {
+			availableCollectors = append(availableCollectors, c)
+		}
+	}
+	return strings.Join(availableCollectors, ",")
+}
+
 func execute(name string, c collector.Collector, ch chan<- prometheus.Metric) {
 	begin := time.Now()
 	err := c.Update(ch)