Make the global Meter useful for registering metric instruments before the SDK is initialized

[jmacd]

There is a stale OTEP here: open-telemetry/oteps#45
It may not describe a real problem in many languages, but it makes a proposal that should at least be followed in Go and probably several other languages.

To make concrete progress in this repository, we should make the global Meter instance a new "forwarding" implementation. It will support allocating metric instruments and label sets before the global instance is registered. It can use a sync.Once object for this purpose. Once the global instance is registered, objects returned by the global instance should begin forwarding to the installed SDK.

Otherwise, metrics allocated on the global Meter implementation are no-op instruments that will remain no-op instruments for the process lifetime.