Replace HashMap with BTreeMap to sort metrics deterministically

[lstrojny]

Addresses #125

[08d2]

Is sorting metrics in this way guaranteed by the spec?

[lstrojny]

To my knowledge the spec doesn’t say anything about ordering so an implementation is free to do whatever it chooses to do. The goal here is not to implement any particular ordering but instead implement some deterministic behavior to enable e.g. reliable automated tests.

[08d2]

If the spec doesn't define deterministic output, then tests can't rely on any output ordering, right? Tests are written not against implementations but against specifications. However, reliable tests can still be achieved, by sorting the output of metric scrapes within the test, and evaluating against the result of those sorts.