Register debug.ReadGCStats, too.
Tests in alphabetical order.
Benchmarks for everyone!
Note about ReadMemStats taking 50-200us.
Add GetOrRegister method for threadsafe metric registration

Advantages over existing metric creation and registration:

* A single statement creates and registers a metric.
* Unused metrics will now be a compiler error (previously all metrics
  would be "used" at least once when registering them).
* The only threadsafe way to dynamically get-or-create metrics without
  wrapping the existing interface.
* Able to get/create metrics locally instead using global variables.
Factor out locked registration and alphabetize.
GetOrRegister for the DefaultRegistry.
New GetOrRegisterCounter API.
Add NewRegistered* for each standard metric.
(#25) Add GetOrRegister* for reach standard metric.
(#25) Get NewRegistered* in on the tests.
Refactor Write to export WriteOnce, too.
WIP big benchmark.
Fanout the metric updaters.
Research, benchmarks, and tests.
More aggressive test.
Direct comparison.
Log sleep time.
Undebug.
Skip tests that require GOMAXPROCS > 1.