Add Aetos datasource

Implement the spec for multi-tenancy support for metrics. This adds
a new 'Aetos' datasource very similar to the current Prometheus
datasource. Because of that, the original PrometheusHelper class
was split into two classes and the base class is used for
PrometheusHelper and for AetosHelper. Except for the split, there
is one more change to the original PrometheusHelper class code, which
is the addition and use of the _get_fqdn_label() and
_get_instance_uuid_label() methods.

As part of the change, I refactored the current prometheus datasource
unit tests. Most of them are now used to test the PrometheusBase class
with minimal changes. Changes I've made to the original tests:

- the ones that can be be used to test the base class are moved into the
  TestPrometheusBase class
- the _setup_prometheus_client, _get_instance_uuid_label and
  _get_fqdn_label functions are mocked in the base class tests.
  Their concrete implementations are tested in each datasource tests
  separately.
- a self._create_helper() is used to instantiate the helper class with
  correct mocking.
- all config value modification is the original tests got moved out and
  instead of modifying the config values, the _get_* methods are mocked
  to return the wanted values
- to keep similar test coverage, config retrieval is tested for each
  concrete class by testing the _get_* methods.

New watcher-aetos-integration and watcher-aetos-integration-realdata
zuul jobs are added to test the new datasource. These use the same set
of tempest tests as the current watcher-prometheus-integration jobs.
The only difference is the environment setup and the Watcher config,
so that the job deploys Aetos and Watcher uses it instead of accessing
Prometheus directly.

At first this was generated by asking cursor to implement the linked spec
with some additional prompts for some smaller changes. Afterwards I manually
went through the code doing some cleanups, ensuring it complies with
PEP8 and hacking and so on. Later on I manually adjusted the code to use
the latest observabilityclient changes.
The zuul job was also mostly generated by cursor.

Implements: https://blueprints.launchpad.net/watcher/+spec/prometheus-multitenancy-support

Generated-By: Cursor with claude-4-sonnet model
Change-Id: I72c2171f72819bbde6c9cbbf565ee895e5d2bd53
Signed-off-by: Jaromir Wysoglad <jwysogla@redhat.com>

releasenotes/notes/aetos-datasource-60e50a2338c64c08.yaml

@@ -0,0 +1,13 @@
+---
+features:
+  - |
+    A new Aetos data source is added. This allows the watcher decision
+    engine to collect metrics through an Aetos reverse proxy server which
+    provides multi-tenant aware access to Prometheus with Keystone
+    authentication and role-based access control. The Aetos datasource
+    uses Keystone service discovery to automatically locate the Aetos
+    endpoint and provides enhanced security compared to direct Prometheus
+    access. For more information about the Aetos data source, including
+    configuration options see
+    https://docs.openstack.org/watcher/latest/datasources/aetos.html
+