Prometheus metrics

Hi,

I've been trying to setup Prometheus scraping metrics out of CodiMD, and use the following dashboard visualizing them:
https://grafana.com/grafana/dashboards/12214

I'm quite new to Prometheus, and find it confusing to have two endpoints (`metrics/router` and `/metrics/codimd`), to scrape metrics from.

FYI, I'm now using the following patch, centralizing all metrics into the same location. I've also introduced a `codimd_` prefix for metrics that used to be served at `/metrics/codimd`, arguably preventing from mixing them up with something else:

```
--- ../codimd/lib/realtime/realtime.js 2020-07-08 21:09:08.120466578 +0200
+++ ./lib/realtime/realtime.js 2020-07-13 11:38:11.744836709 +0200
@@ -9,6 +9,22 @@
const chance = new Chance()
const moment = require('moment')

+// Merging CodiMD metrics into Prometheus main /metrics/router endpoint
+const pclt = require('prom-client');
+const ctrs = [ 'online_notes', 'online_users', 'distinct_online_users',
+ 'notes_count', 'registered_users', 'online_registered_users',
+ 'distinct_online_registered_users', 'is_connection_busy',
+ 'connection_socket_queue_length', 'is_disconnect_busy',
+ 'disconnect_socket_queue_length' ];
+let gauges = [];
+for (let j = 0; j < ctrs.length; j++) {
+ gauges[ctrs[j]] = new pclt.Gauge({
+ name: `codimd_${ctrs[j]}`,
+ help: `CodiMD ${ctrs[j]}`
+ });
+}
+// end-of-patch
+
const get = require('lodash/get')

// core
@@ -282,7 +298,20 @@
})

return models.User.count()
- .then(function (regcount) {
+ .then((regcount) => {
+// Merging CodiMD metrics into Prometheus main /metrics/router endpoint
+ gauges['online_notes'].set(Object.keys(notes).length);
+ gauges['online_users'].set(Object.keys(users).length);
+ gauges['distinct_online_users'].set(distinctaddresses.length);
+ gauges['notes_count'].set(notecount);
+ gauges['registered_users'].set(regcount);
+ gauges['online_registered_users'].set(regaddresses.length);
+ gauges['distinct_online_registered_users'].set(distinctregaddresses.length);
+ gauges['is_connection_busy'].set(connectProcessQueue.lock);
+ gauges['connection_socket_queue_length'].set(connectProcessQueue.queue.length);
+ gauges['is_disconnect_busy'].set(disconnectProcessQueue.lock);
+ gauges['disconnect_socket_queue_length'].set(disconnectProcessQueue.queue.length);
+// end-of-patch
return {
onlineNotes: Object.keys(notes).length,
onlineUsers: Object.keys(users).length,
```

Not sure it really makes sense. I'ld gladly send a PR if it does.

Regards.