Expand on tagvalue option in postgresql_extensible (#3720)

plugins/inputs/postgresql_extensible/README.md

@@ -43,7 +43,11 @@ The example below has two queries are specified, with the following parameters:
   # withdbname was true.
   # Be careful that if the withdbname is set to false you don't have to define
   # the where clause (aka with the dbname)
-  # the tagvalue field is used to define custom tags (separated by comas)
+  #
+  # the tagvalue field is used to define custom tags (separated by comas).
+  # the query is expected to return columns which match the names of the
+  # defined tags. The values in these columns must be of a string-type,
+  # a number-type or a blob-type.
   #
   # Structure :
   # [[inputs.postgresql_extensible.query]]
@@ -109,6 +113,15 @@ using postgreql extensions ([pg_stat_statements](http://www.postgresql.org/docs/
   version=901
   withdbname=false
   tagvalue="db"
+[[inputs.postgresql_extensible.query]]
+  sqlquery="""
+    SELECT type, (enabled || '') AS enabled, COUNT(*)
+      FROM application_users
+      GROUP BY type, enabled
+  """
+  version=901
+  withdbname=false
+  tagvalue="type,enabled"
 ```
 
 # Postgresql Side