Perhaps support pattern (meta) properties

For example,

```json
{"order":"?something","when":"now","#otherProps":false}
```

might mean that any matching message must have no properties other than `order` and `when`. (Presumably such a directive would be local to the structure in which it is present. And presumably not inherited ... )

Or perhaps

```json
{"order":"?something","#expectedTraffic":0.30}
```
might mean that 30 percent of some traffic should match that pattern. In this case, pattern matching would simply ignore the `"#expectedTraffic":0.30`, but a tool looking at the pattern would of course still see that data.