Document new oneOf structure type

Author: badboy

CHANGELOG.md

@@ -4,6 +4,7 @@
 
 * General
   * BREAKING: Remove infill 0 buckets from custom distributions ([#3246](https://github.com/mozilla/glean/pull/3246))
+  * Add a new `oneOf` type to Glean's object metric type structure ([#]())
 * Swift
   * Make `EventMetricType`, `ObjectMetricType`, `URLMetricType` and `Ping` `Sendable`  ([#3255](https://github.com/mozilla/glean/pull/3255))
   * Glean for iOS is now being built with Xcode 16.4 ([#3270](https://github.com/mozilla/glean/pull/3270))

docs/user/reference/metrics/object.md

@@ -218,6 +218,7 @@ The allowed types are:
 * `boolean`
 * `array`
 * `object`
+* `oneOf`
 
 The `array` type takes an `items` parameter, that does define the element types it can hold.  
 The `object` type takes a `properties` parameter, that defines the nested object structure.
@@ -226,6 +227,8 @@ The `object` type takes a `properties` parameter, that defines the nested object
 No other schema parameters are allowed.  
 All fields are optional.
 
+`oneOf` requires a list of alowed subtypes. The available types are `[string, number, boolean]`.
+
 Data is validated against this schema at recording time.  
 Missing values will not be serialized into the payload.