Add discard[] parameter

This PR adds a `discard[]` URL parameter to discard specific enabled collectors.
Compared to `collect[]` parameter, the `discard[]` parameter results in a filtered list which equals enabled collectors minus discarded ones.

A small fix is applied to filtering related debug logs to print the actual slices instead of `unsupported value type`.

Signed-off-by: Siavash Safi <git@hosted.run>

README.md

@@ -339,13 +339,21 @@ mv /path/to/directory/role.prom.$$ /path/to/directory/role.prom
 
 The `node_exporter` will expose all metrics from enabled collectors by default.  This is the recommended way to collect metrics to avoid errors when comparing metrics of different families.
 
-For advanced use the `node_exporter` can be passed an optional list of collectors to filter metrics. The `collect[]` parameter may be used multiple times.  In Prometheus configuration you can use this syntax under the [scrape config](https://prometheus.io/docs/prometheus/latest/configuration/configuration/#<scrape_config>).
+For advanced use the `node_exporter` can be passed an optional list of collectors to filter metrics. The parameters `collect[]` and `discard[]` can be used multiple times (but cannot be combined).  In Prometheus configuration you can use this syntax under the [scrape config](https://prometheus.io/docs/prometheus/latest/configuration/configuration/#<scrape_config>).
 
+Collect only `cpu` and `meminfo` collector metrics:
 ```
   params:
     collect[]:
-      - foo
-      - bar
+      - cpu
+      - meminfo
+```
+
+Collect all enabled collector metrics but discard `netdev`:
+```
+  params:
+    discard[]:
+      - netdev
 ```
 
 This can be useful for having different Prometheus servers collect specific metrics from nodes.

node_exporter.go

@@ -21,6 +21,7 @@ import (
 	"os"
 	"os/user"
 	"runtime"
+	"slices"
 	"sort"
 
 	"github.com/prometheus/common/promlog"
@@ -39,6 +40,9 @@ import (
 	"github.com/prometheus/node_exporter/collector"
 )
 
+// list of enabled collectors used for logging and filtering.
+var enabledCollectors []string
+
 // handler wraps an unfiltered http.Handler but uses a filtered handler,
 // created on the fly, if filtering is requested. Create instances with
 // newHandler.
@@ -75,16 +79,39 @@ func newHandler(includeExporterMetrics bool, maxRequests int, logger log.Logger)
 
 // ServeHTTP implements http.Handler.
 func (h *handler) ServeHTTP(w http.ResponseWriter, r *http.Request) {
-	filters := r.URL.Query()["collect[]"]
-	level.Debug(h.logger).Log("msg", "collect query:", "filters", filters)
+	collects := r.URL.Query()["collect[]"]
+	level.Debug(h.logger).Log("msg", "collect query:", "collects", fmt.Sprintf("%v", collects))
 
-	if len(filters) == 0 {
+	discards := r.URL.Query()["discard[]"]
+	level.Debug(h.logger).Log("msg", "discard query:", "discards", fmt.Sprintf("%v", discards))
+
+	if len(collects) == 0 && len(discards) == 0 {
 		// No filters, use the prepared unfiltered handler.
 		h.unfilteredHandler.ServeHTTP(w, r)
 		return
 	}
+
+	if len(collects) > 0 && len(discards) > 0 {
+		level.Debug(h.logger).Log("msg", "rejecting combined collect and discard queries")
+		w.WriteHeader(http.StatusBadRequest)
+		w.Write([]byte("Combined collect and discard queries are not allowed."))
+		return
+	}
+
+	fileters := &collects
+	if len(discards) > 0 {
+		// In discard mode, filtered collectors = enabled - discarded.
+		f := []string{}
+		for _, c := range enabledCollectors {
+			if (slices.Index(discards, c)) == -1 {
+				f = append(f, c)
+			}
+		}
+		fileters = &f
+	}
+
 	// To serve filtered metrics, we create a filtering handler on the fly.
-	filteredHandler, err := h.innerHandler(filters...)
+	filteredHandler, err := h.innerHandler(*fileters...)
 	if err != nil {
 		level.Warn(h.logger).Log("msg", "Couldn't create filtered metrics handler:", "err", err)
 		w.WriteHeader(http.StatusBadRequest)
@@ -109,12 +136,11 @@ func (h *handler) innerHandler(filters ...string) (http.Handler, error) {
 	// only once upon startup.
 	if len(filters) == 0 {
 		level.Info(h.logger).Log("msg", "Enabled collectors")
-		collectors := []string{}
 		for n := range nc.Collectors {
-			collectors = append(collectors, n)
+			enabledCollectors = append(enabledCollectors, n)
 		}
-		sort.Strings(collectors)
-		for _, c := range collectors {
+		sort.Strings(enabledCollectors)
+		for _, c := range enabledCollectors {
 			level.Info(h.logger).Log("collector", c)
 		}
 	}