metadata: use resource max and end on registration

Ensure the registered resource type does not conflict
with existing resource types or over the max.

Signed-off-by: Derek McGowan <derek@mcg.dev>

metadata/gc.go

@@ -44,6 +44,8 @@ const (
 	ResourceLease
 	// ResourceIngest specifies a content ingest
 	ResourceIngest
+	// resourceEnd is the end of specified resource types
+	resourceEnd
 )
 
 const (