Use metadata server grouping as default

Metadata seems like a better way to group instances for autoscaling, so
switch metadata to be the default option. Note that stable/2024.2 and
older versions don't support metadata grouping, so the jobs are defined
to use prefix grouping instead.

Change-Id: Id1c62255e58e9ceeffad2512a3e8e083c3d13f8b

.zuul.yaml

@@ -63,6 +63,18 @@
     required-projects:
       - name: gnocchixyz/gnocchi
         override-checkout: stable/4.6
+    vars: &prefix_grouping_vars
+      devstack_local_conf:
+        test-config:
+          $TEMPEST_CONFIG:
+            service_available:
+              sg_core: True
+            telemetry_services:
+              metric_backends: gnocchi,prometheus
+            telemetry:
+              disable_ssl_certificate_validation: True
+              ceilometer_polling_interval: 15
+              autoscaling_instance_grouping: prefix
 
 - job:
     name: telemetry-dsvm-integration-2024-1
@@ -71,6 +83,7 @@
     required-projects:
       - name: gnocchixyz/gnocchi
         override-checkout: stable/4.6
+    vars: *prefix_grouping_vars
 
 - job:
     name: telemetry-dsvm-integration-2023-2
@@ -97,6 +110,7 @@
             telemetry:
               disable_ssl_certificate_validation: True
               ceilometer_polling_interval: 15
+              autoscaling_instance_grouping: prefix
 
 - job:
     name: telemetry-dsvm-integration
@@ -107,6 +121,7 @@
     required-projects:
       - name: gnocchixyz/gnocchi
         override-checkout: stable/4.6
+    vars: *prefix_grouping_vars
 
 - job:
     name: telemetry-dsvm-integration
@@ -162,6 +177,7 @@
     required-projects:
       - name: gnocchixyz/gnocchi
         override-checkout: stable/4.6
+    vars: *prefix_grouping_vars
 
 - job:
     name: telemetry-dsvm-integration-ipv6-only

telemetry_tempest_plugin/config.py

@@ -94,14 +94,19 @@ telemetry_opts = [
                default=15,
                help="Scrape interval configured for prometheus. This can "
                     "be used in test cases to properly configure autoscaling"),
-    cfg.StrOpt('autoscaling_instance_grouping',
-               default='prefix',
-               choices=['prefix', 'metadata'],
-               help="How to group instances for autoscaling testing. "
-                    "'prefix' relies on the instances having a common string "
-                    "at the start of their name. 'metadata' is a new and "
-                    "prefered way of grouping since 2024.2 relying on "
-                    "metering.server_group instance metadata")
+    cfg.StrOpt(
+        'autoscaling_instance_grouping',
+        default='metadata',
+        choices=[
+            ('prefix', 'Use a common string at the start of instance names to '
+                       'group instances'),
+            ('metadata', 'Use metering.server_group instance metadata to '
+                         'group instances. This is preferred method since '
+                         '2025.1')
+        ],
+        help="The method to group instances for autoscaling testing. Note "
+             "that this option affects only tests with prometheus metric "
+             "backend")
 ]
 
 telemetry_services_opts = [