Add a metric that tracks max image scaler processes

This will allow us to compute a saturation metric to see
how close we are to exhausting this `actual / max`.

Refs https://gitlab.com/gitlab-com/runbooks/-/issues/52

cable_test.go

@@ -85,9 +85,10 @@ func newUpstreamWithCableConfig(authBackend string, cableBackend string) *config
 	}
 
 	return &config.Config{
-		Version:      "123",
-		DocumentRoot: testDocumentRoot,
-		Backend:      helper.URLMustParse(authBackend),
-		CableBackend: cableBackendURL,
+		Version:            "123",
+		DocumentRoot:       testDocumentRoot,
+		Backend:            helper.URLMustParse(authBackend),
+		CableBackend:       cableBackendURL,
+		ImageResizerConfig: config.DefaultImageResizerConfig,
 	}
 }

changelogs/unreleased/mk-image-scaler-max-procs-metric.yml

+---
+title: Add max_processes Prometheus metric for image scaling
+merge_request: 640
+author:
+type: other

internal/imageresizer/image_resizer.go

@@ -92,7 +92,15 @@ var (
 			Namespace: namespace,
 			Subsystem: subsystem,
 			Name:      "processes",
-			Help:      "Amount of image resizing scaler processes working now",
+			Help:      "Amount of image scaler processes working now",
+		},
+	)
+	imageResizeMaxProcesses = prometheus.NewGauge(
+		prometheus.GaugeOpts{
+			Namespace: namespace,
+			Subsystem: subsystem,
+			Name:      "max_processes",
+			Help:      "The maximum amount of image scaler processes allowed to run concurrently",
 		},
 	)
 	imageResizeRequests = prometheus.NewCounterVec(
@@ -126,11 +134,14 @@ var (
 func init() {
 	prometheus.MustRegister(imageResizeConcurrencyLimitExceeds)
 	prometheus.MustRegister(imageResizeProcesses)
+	prometheus.MustRegister(imageResizeMaxProcesses)
 	prometheus.MustRegister(imageResizeRequests)
 	prometheus.MustRegister(imageResizeDurations)
 }
 
 func NewResizer(cfg config.Config) *Resizer {
+	imageResizeMaxProcesses.Set(float64(cfg.ImageResizerConfig.MaxScalerProcs))
+
 	return &Resizer{Config: cfg, Prefix: "send-scaled-img:"}
 }