Make meter loader work with with meter interface

[c24t]

It looks like #311 broke the meter loader logic. metrics.meter() now always returns a DefaultMeter.

The problem is that loader._load_impl expects api_type to be the type of meter created by the user-supplied factory if they call set_preferred_meter_implementation, and a factory if they don't.

See https://github.com/open-telemetry/opentelemetry-python/pull/341/files#r374382419 for more detail.