exporter: allow custom meter/trace providers

[gdvalle]

This adds a feature to allow passing in a custom trace and metric provider for the self-reporting features.

To do this, alter NewExporter to employ the variadic options pattern over a new options struct, ExporterOptions. Options is already used by the some Config building contraptions.

This does make the package namespace a little crowded... With* functions can apply to either the exporter or the config, but I don't know that makes it confusing enough to split things up yet.