Add a method to initialize a metric to 0 so counters render correctly on their first increment

[weaversam8]

If a counter is not initialized to zero, on the first increment, it will transition from nonexistent to existent with a value of 1. This makes it impossible for a rate(...) or increase(...) query to display any change on the first increment.

[jessestimpson]

Isn't this just simply

imetrics:add(foo, #{tag => bar}, 0)

?

[weaversam8]

Yep. Added #39 to wrap that logic in a function init_counter and add an associated test.