Fix flaky tests for Metricbeat on Windows 2008 R2 (#23490)

* enable 2008

* [CI] Optional stage instead the mandatory one

windows stage is mandatory, but optional stages are not mandatory for PRs

Co-authored-by: Victor Martinez <VictorMartinezRubio@gmail.com>

metricbeat/Jenkinsfile.yml

@@ -62,7 +62,6 @@ stages:
         mage: "mage build unitTest"
         platforms:             ## override default labels in this specific stage.
             - "windows-2019"
-            #- "windows-2008-r2"  https://github.com/elastic/beats/issues/19800
             #- "windows-7-32-bit" https://github.com/elastic/beats/issues/19835
         when:                  ## Override the top-level when.
             not_changeset_full_match: "^x-pack/.*" ## Disable the stage if ONLY changes for the x-pack
@@ -112,6 +111,17 @@ stages:
                 - "windows-8"
             branches: true     ## for all the branches
             tags: true         ## for all the tags
+    windows-2008:
+        mage: "mage build unitTest"
+        platforms:             ## override default labels in this specific stage.
+            - "windows-2008-r2"
+        when:                  ## Override the top-level when.
+            comments:
+                - "/test metricbeat for windows-2008"
+            labels:
+                - "windows-2008"
+            branches: true     ## for all the branches
+            tags: true         ## for all the tags
     windows-7:
         mage: "mage build unitTest"
         platforms:             ## override default labels in this specific stage.