[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/controllers/prometheus_metrics.py

@@ -1,12 +1,52 @@
-# Copyright 2016-2021 Camptocamp SA
+# Copyright 2016 Camptocamp SA
 # License AGPL-3.0 or later (http://www.gnu.org/licenses/agpl.html)
 
-from prometheus_client import generate_latest
+import json
+import logging
 
-from odoo.http import Controller, route
+from prometheus_client import Gauge, generate_latest
+
+from odoo.http import Controller, request, route
+
+_logger = logging.getLogger(__name__)
+
+# Gauges must be instantiated only once per process, registering the same
+# metric name twice raises a "Duplicated timeseries" error.
+_GAUGES = {}
+
+
+def _get_gauge(name, documentation, label_names):
+    gauge = _GAUGES.get(name)
+    if gauge is None:
+        gauge = Gauge(name, documentation, label_names)
+        _GAUGES[name] = gauge
+    return gauge
 
 
 class PrometheusController(Controller):
+    def _publish_gathered_metrics(self):
+        records = request.env["prometheus.metric"].sudo().search([])
+        if not records:
+            return
+
+        cleared = set()
+        for record in records:
+            name = record.name
+            labels = json.loads(record.labels or "{}")
+            gauge = _get_gauge(name, record.documentation or "", sorted(labels))
+            if name not in cleared:
+                # drop the series that disappeared since the last collection
+                gauge.clear()
+                cleared.add(name)
+            if labels:
+                gauge.labels(**labels).set(record.value)
+            else:
+                gauge.set(record.value)
+
     @route("/metrics", auth="public")
     def metrics(self):
+        try:
+            self._publish_gathered_metrics()
+        except Exception:
+            _logger.exception("Could not publish the gathered Prometheus metrics")
         return generate_latest()