Add --web.disable-exporter-metrics flag

If this flag is set, the metrics about the exporter itself (go_*,
process_*, promhttp_*) will be excluded from /metrics.

The Kingpin way of handling boolean flags makes the negative flag
wording (_dis_able) the most reasonably one.

This also refactors the flow in node_exporter.go quite a bit to avoid
mixing up the global and a local registry and to avoid re-creating a
registry even if no filtering is requested.

Signed-off-by: beorn7 <beorn@soundcloud.com>

1 files changed, 95 insertions, 44 deletions

diff --git a/node_exporter.go b/node_exporter.go
index 79863b9..0475a6a 100644
--- a/node_exporter.go
+++ b/node_exporter.go
@@ -27,51 +27,118 @@ import (
         "gopkg.in/alecthomas/kingpin.v2"
 )
 
-func init() {
-        prometheus.MustRegister(version.NewCollector("node_exporter"))
+// handler wraps an unfiltered http.Handler but uses a filtered handler,
+// created on the fly, if filtering is requested. Create instances with
+// newHandler.
+type handler struct {
+        unfilteredHandler http.Handler
+        // exporterMetricsRegistry is a separate registry for the metrics about
+        // the exporter itself.
+        exporterMetricsRegistry *prometheus.Registry
+        includeExporterMetrics  bool
 }
 
-func handler(w http.ResponseWriter, r *http.Request) {
+func newHandler(includeExporterMetrics bool) *handler {
+        h := &handler{
+                exporterMetricsRegistry: prometheus.NewRegistry(),
+                includeExporterMetrics:  includeExporterMetrics,
+        }
+        if h.includeExporterMetrics {
+                h.exporterMetricsRegistry.MustRegister(
+                        prometheus.NewProcessCollector(prometheus.ProcessCollectorOpts{}),
+                        prometheus.NewGoCollector(),
+                )
+        }
+        if innerHandler, err := h.innerHandler(); err != nil {
+                log.Fatalf("Couldn't create metrics handler: %s", err)
+        } else {
+                h.unfilteredHandler = innerHandler
+        }
+        return h
+}
+
+// ServeHTTP implements http.Handler.
+func (h *handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
         filters := r.URL.Query()["collect[]"]
         log.Debugln("collect query:", filters)
 
-        nc, err := collector.NewNodeCollector(filters...)
+        if len(filters) == 0 {
+                // No filters, use the prepared unfiltered handler.
+                h.unfilteredHandler.ServeHTTP(w, r)
+                return
+        }
+        // To serve filtered metrics, we create a filtering handler on the fly.
+        filteredHandler, err := h.innerHandler(filters...)
         if err != nil {
-                log.Warnln("Couldn't create", err)
+                log.Warnln("Couldn't create filtered metrics handler:", err)
                 w.WriteHeader(http.StatusBadRequest)
-                w.Write([]byte(fmt.Sprintf("Couldn't create %s", err)))
+                w.Write([]byte(fmt.Sprintf("Couldn't create filtered metrics handler: %s", err)))
                 return
         }
+        filteredHandler.ServeHTTP(w, r)
+}
 
-        registry := prometheus.NewRegistry()
-        err = registry.Register(nc)
+// innerHandler is used to create buth the one unfiltered http.Handler to be
+// wrapped by the outer handler and also the filtered handlers created on the
+// fly. The former is accomplished by calling innerHandler without any arguments
+// (in which case it will log all the collectors enabled via command-line
+// flags).
+func (h *handler) innerHandler(filters ...string) (http.Handler, error) {
+        nc, err := collector.NewNodeCollector(filters...)
         if err != nil {
-                log.Errorln("Couldn't register collector:", err)
-                w.WriteHeader(http.StatusInternalServerError)
-                w.Write([]byte(fmt.Sprintf("Couldn't register collector: %s", err)))
-                return
+                return nil, fmt.Errorf("couldn't create collector: %s", err)
+        }
+
+        // Only log the creation of an unfiltered handler, which should happen
+        // only once upon startup.
+        if len(filters) == 0 {
+                log.Infof("Enabled collectors:")
+                collectors := []string{}
+                for n := range nc.Collectors {
+                        collectors = append(collectors, n)
+                }
+                sort.Strings(collectors)
+                for _, n := range collectors {
+                        log.Infof(" - %s", n)
+                }
         }
 
-        gatherers := prometheus.Gatherers{
-                prometheus.DefaultGatherer,
-                registry,
+        r := prometheus.NewRegistry()
+        r.MustRegister(version.NewCollector("node_exporter"))
+        if err := r.Register(nc); err != nil {
+                return nil, fmt.Errorf("couldn't register node collector: %s", err)
         }
-        // Delegate http serving to Prometheus client library, which will call collector.Collect.
-        h := promhttp.InstrumentMetricHandler(
-                registry,
-                promhttp.HandlerFor(gatherers,
-                        promhttp.HandlerOpts{
-                                ErrorLog:      log.NewErrorLogger(),
-                                ErrorHandling: promhttp.ContinueOnError,
-                        }),
+        handler := promhttp.HandlerFor(
+                prometheus.Gatherers{h.exporterMetricsRegistry, r},
+                promhttp.HandlerOpts{
+                        ErrorLog:      log.NewErrorLogger(),
+                        ErrorHandling: promhttp.ContinueOnError,
+                },
         )
-        h.ServeHTTP(w, r)
+        if h.includeExporterMetrics {
+                // Note that we have to use h.exporterMetricsRegistry here to
+                // use the same promhttp metrics for all expositions.
+                handler = promhttp.InstrumentMetricHandler(
+                        h.exporterMetricsRegistry, handler,
+                )
+        }
+        return handler, nil
 }
 
 func main() {
         var (
-                listenAddress = kingpin.Flag("web.listen-address", "Address on which to expose metrics and web interface.").Default(":9100").String()
-                metricsPath   = kingpin.Flag("web.telemetry-path", "Path under which to expose metrics.").Default("/metrics").String()
+                listenAddress = kingpin.Flag(
+                        "web.listen-address",
+                        "Address on which to expose metrics and web interface.",
+                ).Default(":9100").String()
+                metricsPath = kingpin.Flag(
+                        "web.telemetry-path",
+                        "Path under which to expose metrics.",
+                ).Default("/metrics").String()
+                disableExporterMetrics = kingpin.Flag(
+                        "web.disable-exporter-metrics",
+                        "Exclude metrics about the exporter itself (promhttp_*, process_*, go_*).",
+                ).Bool()
         )
 
         log.AddFlags(kingpin.CommandLine)
@@ -82,22 +149,7 @@ func main() {
         log.Infoln("Starting node_exporter", version.Info())
         log.Infoln("Build context", version.BuildContext())
 
-        // This instance is only used to check collector creation and logging.
-        nc, err := collector.NewNodeCollector()
-        if err != nil {
-                log.Fatalf("Couldn't create collector: %s", err)
-        }
-        log.Infof("Enabled collectors:")
-        collectors := []string{}
-        for n := range nc.Collectors {
-                collectors = append(collectors, n)
-        }
-        sort.Strings(collectors)
-        for _, n := range collectors {
-                log.Infof(" - %s", n)
-        }
-
-        http.HandleFunc(*metricsPath, handler)
+        http.Handle(*metricsPath, newHandler(!*disableExporterMetrics))
         http.HandleFunc("/", func(w http.ResponseWriter, r *http.Request) {
                 w.Write([]byte(`<html>
                         <head><title>Node Exporter</title></head>
@@ -109,8 +161,7 @@ func main() {
         })
 
         log.Infoln("Listening on", *listenAddress)
-        err = http.ListenAndServe(*listenAddress, nil)
-        if err != nil {
+        if err := http.ListenAndServe(*listenAddress, nil); err != nil {
                 log.Fatal(err)
         }
 }