exporter: rename scrapeall instance label

README.md

@@ -129,14 +129,14 @@ You can use the --split-cluster option to split all cluster nodes into separate
 
 #### Overall targets request endpoint
 
-There is an overall targets endpoint **/scrapeall** that queries all the targets in one request. It can be used to store multiple node metrics without separate target requests. In this case, each node metric will have a **instance** label containing the node name as a host:port pair (or just host if no port was not specified). For example, for mongodb_exporter running with the options:
+There is an overall targets endpoint **/scrapeall** that queries all the targets in one request. It can be used to store multiple node metrics without separate target requests. In this case, each node metric will have a **mongo_instance** label containing the node name as a host:port pair (or just host if no port was not specified). This avoids a collision with Prometheus's reserved `instance` label. For example, for mongodb_exporter running with the options:
 ```
 --mongodb.uri="mongodb://host1:27015,host2:27016" --split-cluster=true
 ``` 
 we get metrics like this:
 ```
-mongodb_up{instance="host1:27015"} 1
-mongodb_up{instance="host2:27016"} 1
+mongodb_up{mongo_instance="host1:27015"} 1
+mongodb_up{mongo_instance="host2:27016"} 1
 ```
 
 #### Enabling collstats metrics gathering

exporter/multi_target_test.go

@@ -105,10 +105,10 @@ func TestOverallHandler(t *testing.T) {
 		},
 	}
 	expected := []*regexp.Regexp{
-		regexp.MustCompile(`mongodb_up{[^\}]*instance="standalone"[^\}]*} 1\n`),
-		regexp.MustCompile(`mongodb_up{[^\}]*instance="s1"[^\}]*} 1\n`),
-		regexp.MustCompile(`mongodb_up{[^\}]*instance="s2"[^\}]*} 1\n`),
-		regexp.MustCompile(`mongodb_up{[^\}]*instance="s3"[^\}]*} 0\n`),
+		regexp.MustCompile(`mongodb_up{[^\}]*mongo_instance="standalone"[^\}]*} 1\n`),
+		regexp.MustCompile(`mongodb_up{[^\}]*mongo_instance="s1"[^\}]*} 1\n`),
+		regexp.MustCompile(`mongodb_up{[^\}]*mongo_instance="s2"[^\}]*} 1\n`),
+		regexp.MustCompile(`mongodb_up{[^\}]*mongo_instance="s3"[^\}]*} 0\n`),
 	}
 	exporters := make([]*Exporter, len(opts))
 

exporter/server.go

@@ -43,6 +43,10 @@ type ServerOpts struct {
 	DisableDefaultRegistry bool
 }
 
+// Label added by /scrapeall to identify the monitored MongoDB target.
+// Use a non-Prometheus-reserved name so Prometheus does not rewrite it.
+const scrapeAllInstanceLabel = "mongo_instance"
+
 // RunWebServer runs the main web-server
 func RunWebServer(opts *ServerOpts, exporters []*Exporter, log *slog.Logger) {
 	mux := http.NewServeMux()
@@ -104,7 +108,7 @@ func multiTargetHandler(serverMap ServerMap) http.HandlerFunc {
 }
 
 // OverallTargetsHandler is a handler to scrape all the targets in one request.
-// Adds instance label to each metric.
+// Adds a MongoDB-target label to each metric.
 func OverallTargetsHandler(exporters []*Exporter, logger *slog.Logger) http.HandlerFunc {
 	return func(w http.ResponseWriter, r *http.Request) {
 		seconds, err := strconv.Atoi(r.Header.Get("X-Prometheus-Scrape-Timeout-Seconds"))
@@ -153,12 +157,7 @@ func OverallTargetsHandler(exporters []*Exporter, logger *slog.Logger) http.Hand
 				registry.MustRegister(gc)
 			}
 
-			hostlabels := prometheus.Labels{}
-			if e.opts.NodeName != "" {
-				hostlabels["instance"] = e.opts.NodeName
-			}
-
-			gw := NewGathererWrapper(registry, hostlabels)
+			gw := NewGathererWrapper(registry, scrapeAllHostLabels(e.opts.NodeName))
 			gatherers = append(gatherers, gw)
 		}
 
@@ -172,6 +171,14 @@ func OverallTargetsHandler(exporters []*Exporter, logger *slog.Logger) http.Hand
 	}
 }
 
+func scrapeAllHostLabels(nodeName string) prometheus.Labels {
+	hostlabels := prometheus.Labels{}
+	if nodeName != "" {
+		hostlabels[scrapeAllInstanceLabel] = nodeName
+	}
+	return hostlabels
+}
+
 func buildServerMap(exporters []*Exporter, log *slog.Logger) ServerMap {
 	servers := make(ServerMap, len(exporters))
 	for _, e := range exporters {

exporter/server_test.go

@@ -0,0 +1,30 @@
+// mongodb_exporter
+// Copyright (C) 2017 Percona LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package exporter
+
+import (
+	"testing"
+
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/stretchr/testify/assert"
+)
+
+func TestScrapeAllHostLabels(t *testing.T) {
+	t.Parallel()
+
+	assert.Equal(t, prometheus.Labels{}, scrapeAllHostLabels(""))
+	assert.Equal(t, prometheus.Labels{"mongo_instance": "node-a:27017"}, scrapeAllHostLabels("node-a:27017"))
+}