changelog update: log options & dirty_pages metric
## [Unreleased]
+### Added
+- The `/logger` API has a new field called `options`. This is an array of
+ strings that specify additional logging configurations. The only supported
+ value is `LogDirtyPages`.
+- When the `LogDirtyPages` option is configured via `PUT /logger`, a new metric
+ called `memory.dirty_pages` is computed as the number of pages dirtied by the
+ guest since the last time the metric was flushed.
+
### Changed
- `PUT` requests on `/mmds` always return 204 on success.
