Backend: Set content-type on metrics endpoints (#5042)

The prometheus text format requires metrics endpoints respond with the
content-type 'text/plain; version=0.0.4'. Without this, newer versions
of prometheus fail to scrape the metrics endpoint and report an error.
It's possible to work around this by setting the
'fallback_scrape_protocol' setting in the prometheus scrape target
configuration, but this revision sets the content type appropriately to
avoid this in the first place.

internal/api/metrics.go

@@ -14,6 +14,7 @@ import (
 	"github.com/photoprism/photoprism/internal/auth/acl"
 	"github.com/photoprism/photoprism/internal/config"
 	"github.com/photoprism/photoprism/internal/photoprism/get"
+	"github.com/photoprism/photoprism/pkg/media/http/header"
 )
 
 // GetMetrics provides a prometheus-compatible metrics endpoint for monitoring.
@@ -31,6 +32,8 @@ func GetMetrics(router *gin.RouterGroup) {
 		conf := get.Config()
 		counts := conf.ClientUser(false).Count
 
+		header.SetContentType(c.Request, header.ContentTypePrometheus)
+
 		c.Stream(func(w io.Writer) bool {
 			reg := prometheus.NewRegistry()
 			reg.MustRegister(collectors.NewGoCollector())

internal/api/metrics_test.go

@@ -29,6 +29,7 @@ func TestGetMetrics(t *testing.T) {
 		assert.Regexp(t, regexp.MustCompile(`photoprism_statistics_media_count{stat="folders"} \d+`), body)
 		assert.Regexp(t, regexp.MustCompile(`photoprism_statistics_media_count{stat="files"} \d+`), body)
 	})
+
 	t.Run("expose build information", func(t *testing.T) {
 		app, router, _ := NewApiTest()
 
@@ -44,4 +45,18 @@ func TestGetMetrics(t *testing.T) {
 
 		assert.Regexp(t, regexp.MustCompile(`photoprism_build_info{edition=".+",goversion=".+",version=".+"} 1`), body)
 	})
+
+	t.Run("has prometheus exposition format as content type", func(t *testing.T) {
+		app, router, _ := NewApiTest()
+
+		GetMetrics(router)
+
+		resp := PerformRequestWithStream(app, "GET", "/api/v1/metrics")
+		if resp.Code != http.StatusOK {
+			t.Fatal(resp.Body.String())
+		}
+		if contentType := resp.Result().Header.Get("Content-Type"); contentType != "" {
+			t.Fatalf("unexpected response content-type: %s", contentType)
+		}
+	})
 }

pkg/media/http/header/content_types.go

@@ -106,6 +106,7 @@ const (
 	ContentTypeText       = "text/plain; charset=utf-8"
 	ContentTypePDF        = "application/pdf"
 	ContentTypeZip        = "application/zip"
+	ContentTypePrometheus = "text/plain; version=0.0.4"
 )
 
 // HasContentType checks weather the Content-Type header has the specified type.