[ADD] monitoring_prometheus_queue_job

This commit features a refactoring of monitoring_prometheus to get a
cron gathering some metrics from the database, and then publishing the
result on the /metrics endpoint

The new module monitoring_prometheus_queue_job extends the cron to get
information about the queue job states.

monitoring_prometheus/README.rst

@@ -15,3 +15,29 @@ Current available metrics are:
 * Longpolling request count
 
 No additional configuration is needed, just ensure that the Prometheus server is allowed to communicate with Odoo
+
+Gathering metrics from the database
+-----------------------------------
+
+Metrics that require a database query are collected by the *Prometheus: gather metrics*
+scheduled action, which runs every 15 minutes and stores its result as
+``prometheus.metric`` records (one per metric name/labels combination). The */metrics*
+endpoint reads those records and publishes them as gauges. This indirection is required
+because the cron and the HTTP workers run in separate processes and cannot share the
+Prometheus registry.
+
+By default, only users in the *Administration / Settings* group (``base.group_system``)
+can read ``prometheus.metric`` records through the ORM/UI. The */metrics* endpoint
+remains public and reads the records with ``sudo()`` so Prometheus can keep scraping it
+without authentication.
+
+To publish new metrics, extend ``_gather_metrics`` on the ``prometheus.gatherer``
+abstract model and return the result of ``super()`` extended with your own entries::
+
+    {
+        "name": "odoo_some_metric",
+        "documentation": "What this metric measures",
+        "labels": {"label_name": "label_value"},
+        "value": 42,
+    }
+