Add descriptions to metric macro invocation

[Morrigan Haughey]

We should allow the user to describe a metric when defining it. This would be useful for users of this library because it would permit them to add CLI flags like --stats to dump information about the static metrics being recorded.

the idea being to permit a metric definition like so:

#[metric(
    name = "view-counter",
    description = "counts the number of views to our website"
)]
static VIEW_COUNTER: Counter = Counter::new();