Allow configuring Prometheus port

[perlfisch]

Prometheus is currently hard coded to start on port 9090. I already have JBV running on this port, which is it's documented default, so excalidraw-backend fails to start.

Obviously I can change the port of JVB, but the ability to configure the prometheus port or allow disabling prometheus entirely would be nice.

[RustyDust]

For a quick fix use:

--- src/index.ts.org	2023-06-14 16:11:09
+++ src/index.ts	2023-06-14 16:08:21
@@ -43,7 +43,8 @@
 // listens on host:9090/metrics
 prometheus.metrics(io, {
-    collectDefaultMetrics: true
+    collectDefaultMetrics: true,
+    port: 9190
 });

And change the port to what you like it to be.