Initialize all metrics to zero

[emschwartz]

@mies made a good point that it would be useful to have all the metrics start out at zero so you'd see them appear in external systems pulling from Prometheus even before the app has seen any traffic

[emschwartz]

I think we can do this in the following way:

1. When the autometrics macro is expanded at compile time, generate code that uses linkme to collect all of the function details into a "distributed slice" that's available at compile time
2. Have a function that uses the aforementioned collection of function details to set all the metrics to zero (by adding zero to the counters)
3. Call said function when the global metrics exporter is initialized

In order to avoid adding to the binary size for production applications for this, we can have all of this functionality behind a config flag that only applies when it's in debug mode. I think it's safe to assume that you only care about zeroing out the metrics in debug mode when you might be using something like the admin panel.