Implement "Named Tracers and Meters"

[z1c0]

The "Named Tracers and Meters" RFC (https://github.com/open-telemetry/oteps/blob/master/text/0016-named-tracers.md) has been approved and was added to the specification (open-telemetry/opentelemetry-specification#264).

Please make sure to also update the documentation if necessary (tracer, meter creation).

OpenTelemetry language repositories now need to implement this mechanisms.

[z1c0]

There is a similarly named issue already #127, but since it was created way before the spec changes were accepted, this is probably something different.

[jmacd]

@rghetia is working on this.