Document the default value for send_in_pings

[doc only]

docs/user/reference/yaml/metrics.md

@@ -189,11 +189,20 @@ Defines the lifetime of the metric. Different lifetimes affect when the metrics
 _default: `events`|`metrics`_
 
 Defines which pings the metric should be sent on.
-If not specified, the metric is sent on the "default ping",
+If not specified, the metric is sent on the default ping,
 which is the `events` ping for events and the `metrics` ping for everything else.
 
 Most metrics don't need to specify this unless they are sent on [custom pings](../../user/pings/custom.md).
 
+The special value `default` may be used, in case it's required for a metric to be sent
+on the default ping as well as in a custom ping.
+
+```yaml
+send_in_pings:
+  - my-custom-ping
+  - default
+```
+
 #### `disabled`
 
 _default: `false`_

docs/user/reference/yaml/pings.md

@@ -63,7 +63,7 @@ ping names on generated code.
 
 ### Reserved ping names
 
-> The names `baseline`, `metrics`, `events`, `deletion-request` and `all-pings` are reserved
+> The names `baseline`, `metrics`, `events`, `deletion-request`, `default` and `all-pings` are reserved
 > and may not be used as the name of a custom ping.
 
 ## Ping parameters