chore(fs): changes to allow Filesystem to be implemented externally (#10040)

### Purpose

The `fs.Filesystem` interface contains two parts that cannot be
implemented externally because they are private:

* `filesystemWrapperType`: this PR changes `unwrapFilesystem` to
downcast to a specific concrete type
* `underlying`: this PR simply moves it to an unexported interface

### Testing

Regular tests pass.

lib/fs/metrics.go

@@ -273,10 +273,6 @@ func (m *metricsFS) underlying() (Filesystem, bool) {
 	return m.next, true
 }
 
-func (m *metricsFS) wrapperType() filesystemWrapperType {
-	return filesystemWrapperTypeMetrics
-}
-
 type metricsFile struct {
 	fs   *metricsFS
 	next File