Add ability to add custom span processors and metric readers

[lzchen]

Note we did not include configuring LogRecordProcessors because logging is unstable. Asking the user to specify those classes may induce breaking changes.

Fixes #236

[lzchen]

The idea is to suggest users to use opentelemetry apis to achieve this functionality:

get_tracer_provider().add_span_processor().

Metrics doesn't have the add_metric_reader() functionality due to setup of measurement consumer callbacks, so we will probably only expose this for metrics.