docs: add dogstatsd_tags example to description

Clarify the expected format with a concrete example.

Closes #3288

gunicorn/config.py

@@ -1672,7 +1672,7 @@ class DogstatsdTags(Setting):
     validator = validate_string
     desc = """\
     A comma-delimited list of datadog statsd (dogstatsd) tags to append to
-    statsd metrics.
+    statsd metrics. e.g. ``'tag1:value1,tag2:value2'``
 
     .. versionadded:: 20
     """