Add prometheus metric for image builds started

Author: Andrew Farries

Diff for: components/server/src/prometheus-metrics.ts

@@ -21,6 +21,7 @@ export function registerServerMetrics(registry: prometheusClient.Registry) {
     registry.registerMetric(instanceStartsFailedTotal);
     registry.registerMetric(prebuildsStartedTotal);
     registry.registerMetric(stripeClientRequestsCompletedDurationSeconds);
+    registry.registerMetric(imageBuildsStartedTotal);
 }
 
 const loginCounter = new prometheusClient.Counter({
@@ -176,3 +177,12 @@ export const stripeClientRequestsCompletedDurationSeconds = new prometheusClient
 export function observeStripeClientRequestsCompleted(operation: string, outcome: string, durationInSeconds: number) {
     stripeClientRequestsCompletedDurationSeconds.observe({ operation, outcome }, durationInSeconds);
 }
+
+export const imageBuildsStartedTotal = new prometheusClient.Counter({
+    name: "gitpod_server_image_builds_started_total",
+    help: "counter of the total number of image builds started on server",
+});
+
+export function increaseImageBuildsStartedTotal() {
+    imageBuildsStartedTotal.inc();
+}