unmashall failure

The current conversion of GO`*Expr` structs to JSON is failing to convert values with complex queries, ie:
```
node example.js 'histogram_quantile(0.9, sum(rate(traces_service_graph_request_server_seconds_bucket{server="api"}[1m])) by (le, client, server))'
```

Short queries in the examples works:
```
node example.js `sum(rate(foo{bar="baz"}[5m])) by (job)`
```