Use a sorted list for attributes in metrics

[tsloughter]

Because a key in a matchspec acts as a match and not an equality check the ets lookups based on keys using attributes in metrics will result in incorrect results. For example, an empty set of attributes #{} would match any metric, no matter the attributes it has, when used in an ets match/select.

I believe moving to a guard check will slow the key lookup down so we must convert attributes to a list when used in a metric key.

[tsloughter]

Renaming this because the more important issue issue is normalizing attribute keys since right now a and <<"a">> would make different metric streams.