Make prometheus the default devstack example

Change the devstack local.conf samples and devstack multinode
contributor doc to demonstrate deploying watcher with prometheus as
datasource instead of gnocchi. Keep the gnocchi as an alternative
deployment example.

Depends-On: https://review.opendev.org/c/openstack/watcher/+/946230
Depends-On: https://review.opendev.org/c/openstack/devstack-plugin-prometheus/+/946254

Change-Id: I721b550a03f9e5350a3f1ab10292faa1c50049a7

doc/source/contributor/devstack.rst

@@ -31,14 +31,45 @@ Quick Devstack Instructions with Datasources
 ============================================
 
 Watcher requires a datasource to collect metrics from compute nodes and
-instances in order to execute most strategies. To enable this a
-``[[local|localrc]]`` to setup DevStack for some of the supported datasources
-is provided. These examples specify the minimal configuration parameters to
-get both Watcher and the datasource working but can be expanded is desired.
+instances in order to execute most strategies. To enable this two possible
+examples of ``[[local|localrc]]`` to setup DevStack for some of the
+supported datasources is provided. These examples specify the minimal
+configuration parameters to get both Watcher and the datasource working
+but can be expanded is desired.
+The first example configures watcher to user prometheus as a datasource, while
+the second example show how to use gnocchi as the datasource. The procedure is
+equivalent, it just requires using the ``local.conf.controller`` and
+``local.conf.compute`` in the first example and
+``local_gnocchi.conf.controller`` and ``local_gnocchi.conf.compute`` in the
+second.
+
+Prometheus
+----------
+
+With the Prometheus datasource most of the metrics for compute nodes and
+instances will work with the provided configuration but metrics that
+require Ironic such as ``host_airflow and`` ``host_power`` will still be
+unavailable as well as ``instance_l3_cpu_cache``
+
+.. code-block:: ini
+
+   [[local|localrc]]
+
+   enable_plugin watcher https://opendev.org/openstack/watcher
+   enable_plugin watcher-dashboard https://opendev.org/openstack/watcher-dashboard
+   enable_plugin ceilometer https://opendev.org/openstack/ceilometer.git
+   enable_plugin aodh https://opendev.org/openstack/aodh
+   enable_plugin devstack-plugin-prometheus https://opendev.org/openstack/devstack-plugin-prometheus
+   enable_plugin sg-core https://github.com/openstack-k8s-operators/sg-core main
+
+
+   CEILOMETER_BACKEND=sg-core
+   [[post-config|$NOVA_CONF]]
+   [DEFAULT]
+   compute_monitors=cpu.virt_driver
 
 Gnocchi
 -------
-
 With the Gnocchi datasource most of the metrics for compute nodes and
 instances will work with the provided configuration but metrics that
 require Ironic such as ``host_airflow and`` ``host_power`` will still be
@@ -96,7 +127,8 @@ Detailed DevStack Instructions
         cd ~
         git clone https://opendev.org/openstack/devstack.git
 
-#. For each compute node, copy the provided `local.conf.compute`_ example file
+#. For each compute node, copy the provided `local.conf.compute`_
+   (`local_gnocchi.conf.compute`_ if deploying with gnocchi) example file
    to the compute node's system at ~/devstack/local.conf. Make sure the
    HOST_IP and SERVICE_HOST values are changed appropriately - i.e., HOST_IP
    is set to the IP address of the compute node and SERVICE_HOST is set to the
@@ -112,7 +144,8 @@ Detailed DevStack Instructions
    to configure similar configuration options for the projects providing those
    metrics.
 
-#. For the controller node, copy the provided `local.conf.controller`_ example
+#. For the controller node, copy the provided `local.conf.controller`_
+   (`local_gnocchi.conf.controller`_ if deploying with gnocchi) example
    file to the controller node's system at ~/devstack/local.conf. Make sure
    the HOST_IP value is changed appropriately - i.e., HOST_IP is set to the IP
    address of the controller node.
@@ -142,6 +175,17 @@ Detailed DevStack Instructions
         to FALSE. For Production environment it is suggested to keep it at the
         default TRUE value.
 
+#. If you want to use prometheus as a datasource, you need to provide a
+   Prometheus configuration with the compute nodes set as targets, so
+   it can consume their node-exporter metrics (if you are deploying watcher
+   with gnocchi as datasource you can skip this step altogether). Copy the
+   provided `prometheus.yml`_ example file and set the appropriate hostnames
+   for all the compute nodes (the example configures 2 of them plus the
+   controller, but you should add all of them if using more than 2 compute
+   nodes). Set the value of ``PROMETHEUS_CONFIG_FILE`` to the path of the
+   file you created in the local.conf file (the sample local.conf file uses
+   ``$DEST`` as the default value for the prometheus config path).
+
 #. Start stacking from the controller node::
 
        ./devstack/stack.sh
@@ -154,6 +198,9 @@ Detailed DevStack Instructions
 
 .. _local.conf.controller: https://github.com/openstack/watcher/tree/master/devstack/local.conf.controller
 .. _local.conf.compute: https://github.com/openstack/watcher/tree/master/devstack/local.conf.compute
+.. _local_gnocchi.conf.controller: https://github.com/openstack/watcher/tree/master/devstack/local_gnocchi.conf.controller
+.. _local_gnocchi.conf.compute: https://github.com/openstack/watcher/tree/master/devstack/local_gnocchi.conf.compute
+.. _prometheus.yml: https://github.com/openstack/watcher/tree/master/devstack/prometheus.yml
 
 Multi-Node DevStack Environment
 ===============================