[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/data/ir_cron.xml

@@ -0,0 +1,16 @@
+<?xml version="1.0" encoding="utf-8" ?>
+<odoo>
+    <data noupdate="1">
+        <record id="cron_prometheus_gather_metrics" model="ir.cron">
+            <field name="name">Prometheus: gather metrics</field>
+            <field name="model_id" ref="model_prometheus_gatherer" />
+            <field name="state">code</field>
+            <field name="code">model._cron_gather_metrics()</field>
+            <field name="interval_number">15</field>
+            <field name="interval_type">minutes</field>
+            <field name="numbercall">-1</field>
+            <field name="doall" eval="False" />
+            <field name="active" eval="True" />
+        </record>
+    </data>
+</odoo>