Disable SortMapKeys in query-frontend response encoding

[pracucci]

The query-frontend Prometheus query range response encoding (json) has SortMapKeys enabled. Based on the comment received on 3964, we could try to disable it, to have the json encoding having to do less work.

Questions:

1. What's the impact on performances? If there's no gain, then there's probably no value into risk a change.
2. Does it break any client? What's about Grafana?
3. Does Prometheus guarantees sorted keys?