Deprecate Monasca data source

The Monasca project was marked inactive during 2023.1. Although we have
seen multiple people showing interest to keep the project, we haven't
seen any real progress.

Because the project is likely retired soon, let's deprecate the feature
dependent on Monasca so that we can remove it in a future release.

Change-Id: Ifd64f5ba59bbac238ff62302ec36a3e36954d6d0

releasenotes/notes/deprecate-monasca-ds-9065f4d4bee09ab2.yaml

@@ -0,0 +1,5 @@
+---
+deprecations:
+  - |
+    Monasca Data Source is deprecated and will be removed in the future, due
+    to inactivity of Monasca project.

watcher/decision_engine/datasources/manager.py

@@ -63,6 +63,9 @@ class DataSourceManager(object):
                 LOG.warning('Invalid Datasource: %s. Allowed: %s ', *msgargs)
 
         self.datasources = self.config.datasources
+        if self.datasources and mon.MonascaHelper.NAME in self.datasources:
+            LOG.warning('The monasca datasource is deprecated and will be '
+                        'removed in a future release.')
 
     @property
     def monasca(self):