Namespace instruments by Meter

[tsloughter]

The name of an instrument must be namespaced by the Meter it was created with:

https://github.com/open-telemetry/opentelemetry-specification/blob/master/specification/metrics/api.md#instrument-naming-requirements

Currently the name of the meter is ignored by new_instrument in otel_meter_default.

Obviously this also means that when looking up an instrument the name of the Meter must be included.