Clear gauges after scrape (#3)

* WEB-2117 wipe gauges after scrape
---------
Co-authored-by: Pedro Ladaria <[email protected]>

CONTRIBUTING.md

@@ -0,0 +1,64 @@
+# Contributing
+
+## Install Go
+
+https://golang.org/doc/install
+
+## Install dependencies
+
+```sh
+go mod tidy
+```
+
+## Run
+
+```sh
+go run . --apiListen 127.0.0.1:8080
+```
+
+## Send some metrics
+
+```
+echo "some_metric 3.14" | curl --data-binary @- http://127.0.0.1:8080/metrics/job/some_job
+
+printf "#TYPE another_metric gauge\nanother_metric 42\n" | curl --data-binary @- http://127.0.0.1:8080/metrics/job/some_job
+```
+
+## See your metric
+
+Open http://127.0.0.1:8080/metrics in your browser or use `curl`:
+
+```sh
+curl http://127.0.0.1:8080/metrics
+```
+
+Expected result
+
+```
+# TYPE another_metric gauge
+another_metric{job="some_job"} 42
+# TYPE some_metric untyped
+some_metric{job="some_job"} 3.14
+```
+
+## Simulate a scrape from Prometheus
+
+-   With "Prometheus/1.0" as user-agent
+-   Gauges will be cleared after the scrape
+
+```sh
+curl -H "User-Agent: Prometheus/1.0" http://127.0.0.1:8080/metrics
+```
+
+Returns the same as above. But if executed again, `gauge` metrics are cleared:
+
+```
+# TYPE some_metric untyped
+some_metric{job="some_job"} 3.14
+```
+
+## Run tests
+
+```sh
+go test ./...
+```

metrics/aggregate.go

@@ -149,7 +149,22 @@ func (a *Aggregate) HandleRender(c *gin.Context) {
 	c.Header("Content-Type", string(contentType))
 	a.encodeAllMetrics(c.Writer, contentType)
 
-	// TODO reset gauges
+	// Remove gauge metrics after serving the response to Prometheus
+	userAgent := c.Request.Header.Get("User-Agent")
+	if strings.Contains(userAgent, "Prometheus/") {
+		a.removeGaugeMetrics()
+	}
+}
+
+func (a *Aggregate) removeGaugeMetrics() {
+	a.familiesLock.Lock()
+	defer a.familiesLock.Unlock()
+
+	for name, family := range a.families {
+		if family.GetType() == dto.MetricType_GAUGE {
+			delete(a.families, name)
+		}
+	}
 }
 
 func (a *Aggregate) encodeAllMetrics(writer io.Writer, contentType expfmt.Format) {

metrics/aggregate_test.go

@@ -4,9 +4,12 @@ import (
 	"bytes"
 	"context"
 	"fmt"
+	"net/http"
+	"net/http/httptest"
 	"strings"
 	"testing"
 
+	"github.com/gin-gonic/gin"
 	"github.com/pmezard/go-difflib/difflib"
 	"github.com/prometheus/common/expfmt"
 	"github.com/stretchr/testify/require"
@@ -65,7 +68,7 @@ histogram_count 1
 counter{job="test"} 60
 # HELP gauge A gauge
 # TYPE gauge gauge
-gauge{job="test"} 99
+gauge{job="test"} 57
 # HELP histogram A histogram
 # TYPE histogram histogram
 histogram_bucket{job="test",le="1"} 0
@@ -118,9 +121,9 @@ ui_external_lib_loaded{name="mixpanel",loaded="true"} 1
 `
 	gaugeOutput = `# HELP ui_external_lib_loaded A gauge with entries in un-sorted order
 # TYPE ui_external_lib_loaded gauge
-ui_external_lib_loaded{job="test",loaded="true",name="Intercom"} 2
-ui_external_lib_loaded{job="test",loaded="true",name="ga"} 2
-ui_external_lib_loaded{job="test",loaded="true",name="mixpanel"} 2
+ui_external_lib_loaded{job="test",loaded="true",name="Intercom"} 1
+ui_external_lib_loaded{job="test",loaded="true",name="ga"} 1
+ui_external_lib_loaded{job="test",loaded="true",name="mixpanel"} 1
 `
 	duplicateLabels = `
 # HELP ui_external_lib_loaded Test with duplicate values
@@ -277,3 +280,47 @@ func BenchmarkConcurrentAggregate(b *testing.B) {
 		})
 	}
 }
+
+func TestRemoveGaugeMetrics(t *testing.T) {
+	a := NewAggregate()
+
+	// Add some metrics to the aggregate
+	a.parseAndMerge(strings.NewReader(`# TYPE gauge_metric gauge
+gauge_metric{label="value"} 123
+# TYPE counter_metric counter
+counter_metric{label="value"} 456
+`), nil)
+
+	// Simulate a scrape with Prometheus user-agent
+	req, err := http.NewRequest("GET", "/metrics", nil)
+	if err != nil {
+		t.Fatalf("unexpected error creating request: %s", err)
+	}
+	req.Header.Set("User-Agent", "Prometheus/2.0")
+
+	// Create a response recorder
+	rr := httptest.NewRecorder()
+
+	// Handle the request
+	router := gin.Default()
+	router.GET("/metrics", a.HandleRender)
+	router.ServeHTTP(rr, req)
+
+	// Check the response code
+	if status := rr.Code; status != http.StatusOK {
+		t.Errorf("handler returned wrong status code: got %v want %v", status, http.StatusOK)
+	}
+
+	// Check that the gauge metric has been removed
+	a.familiesLock.RLock()
+	defer a.familiesLock.RUnlock()
+
+	if _, exists := a.families["gauge_metric"]; exists {
+		t.Errorf("gauge_metric was not removed")
+	}
+
+	// Check that the counter metric still exists
+	if _, exists := a.families["counter_metric"]; !exists {
+		t.Errorf("counter_metric was removed but it should not have been")
+	}
+}