config/statistics: Add discrete modifiers

data/updates.js

@@ -48,6 +48,7 @@ export const updates = {
 	mail_crypt_fs_maybe: '2.4.0',
 	mail_location_keep_noselect: '2.4.0',
 	mail_location_no_noselect: '2.4.0',
+	metric_group_by_discrete_modifiers_added: '2.4.0',
 	migration_imapc_features: '2.4.0',
 	pbkdf2_hashing: '2.4.0',
 	process_title_imap_process: '2.4.0',

docs/core/config/statistics.md

@@ -79,13 +79,26 @@ is a very common use case, not specifying an aggregation function is treated
 as an alias for discrete aggregation.  In other words, `field` and
 `field:discrete` produce the same behavior.
 
+[[added,metric_group_by_discrete_modifiers_added]] An additional parameter
+can be added to provide modifiers to the discrete value. This is done as
+[[link,settings_variables,%variables]] and their modifiers. The following
+variables are provided:
+
+* `%{value}` - The original value
+* `%{domain}` - Text after the `@` character, or empty string if there is no `@`.
+
 Example:
 
 ```[dovecot.conf]
 metric imap_command {
   filter = event=imap_command_finished
   group_by = cmd_name tagged_reply_state
 }
+metric login_domains {
+  filter = event=auth_request_finished
+  fields = user
+  group_by = discrete:%L{domain}
+}
 ```
 
 This example configuration will generate statistics for each IMAP command.