Add custom prometheus metrics

This reuses the `realtime.getStatus` method to get the state of the
application state on every prometheus scrape cycle.

Signed-off-by: David Mehren <git@herrmehren.de>

app.js

@@ -26,6 +26,7 @@ const logger = require('./lib/logger')
 const errors = require('./lib/errors')
 const models = require('./lib/models')
 const csp = require('./lib/csp')
+const metrics = require('./lib/prometheus')
 
 // server setup
 const app = express()
@@ -66,6 +67,7 @@ app.use(morgan('combined', {
 
 // Register prometheus metrics endpoint
 app.use(apiMetrics())
+metrics.setupCustomPrometheusMetrics()
 
 // socket io
 const io = require('socket.io')(server, { cookie: false })

lib/prometheus.js

@@ -0,0 +1,49 @@
+const promClient = require('prom-client')
+const realtime = require('./realtime')
+
+exports.setupCustomPrometheusMetrics = function () {
+  const onlineNotes = new promClient.Gauge({
+    name: 'hedgedoc_online_notes',
+    help: 'Notes currently being edited'
+  })
+  const onlineSessions = new promClient.Gauge({
+    name: 'hedgedoc_online_sessions',
+    help: 'Sessions currently editing notes',
+    labelNames: ['type']
+  })
+  const onlineUsers = new promClient.Gauge({
+    name: 'hedgedoc_online_users',
+    help: 'Distinct users currently editing notes',
+    labelNames: ['type']
+  })
+  const notesCount = new promClient.Gauge({
+    name: 'hedgedoc_notes',
+    help: 'Notes in the instance'
+  })
+  const registeredUsers = new promClient.Gauge({
+    name: 'hedgedoc_registered_users',
+    help: 'Users that registered in the instance'
+  })
+  const isConnectionBusy = new promClient.Gauge({
+    name: 'hedgedoc_connection_busy',
+    help: 'Indicates that realtime currently connecting'
+  })
+  const connectionSocketQueueLength = new promClient.Gauge({
+    name: 'hedgedoc_connection_socket_queue_length',
+    help: 'Length of connection socket queue',
+    // The last gauge provides the collect callback for all metrics
+    collect () {
+      realtime.getStatus(function (data) {
+        onlineNotes.set(data.onlineNotes)
+        onlineSessions.set({ type: 'all' }, data.onlineUsers)
+        onlineSessions.set({ type: 'signed-in' }, data.onlineRegisteredUsers)
+        onlineUsers.set({ type: 'all' }, data.distinctOnlineUsers)
+        onlineUsers.set({ type: 'signed-in' }, data.distinctOnlineRegisteredUsers)
+        notesCount.set(data.notesCount)
+        registeredUsers.set(data.registeredUsers)
+        isConnectionBusy.set(data.isConnectionBusy ? 1 : 0)
+        connectionSocketQueueLength.set(data.connectionSocketQueueLength)
+      })
+    }
+  })
+}