Fix fields validation in non utf-8 environments (#18130) (#18136)

There was a non-ascii character in fields documentation that was making
fields validation in some tests to fail.

Fix fields validation to don't fail on this case, and replace the non-ascii
character.

(cherry picked from commit 93b5998)

libbeat/tests/system/beat/beat.py

@@ -586,7 +586,7 @@ def extract_fields(doc_list, name):
 
         # TODO: Make fields_doc path more generic to work with beat-generator. If it can't find file
         # "fields.yml" you should run "make update" on metricbeat folder
-        with open(fields_doc, "r") as f:
+        with open(fields_doc, "r", encoding="utf_8") as f:
             path = os.path.abspath(os.path.dirname(__file__) + "../../../../fields.yml")
             if not os.path.isfile(path):
                 path = os.path.abspath(os.path.dirname(__file__) + "../../../../_meta/fields.common.yml")

metricbeat/docs/fields.asciidoc

@@ -4168,7 +4168,7 @@ type: long
 *`aws.sns.metrics.NumberOfNotificationsFilteredOut-InvalidAttributes.sum`*::
 +
 --
-The number of messages that were rejected by subscription filter policies because the messages' attributes are invalid – for example, because the attribute JSON is incorrectly formatted.
+The number of messages that were rejected by subscription filter policies because the messages' attributes are invalid - for example, because the attribute JSON is incorrectly formatted.
 
 type: long