Add start_pipeline to MeterProvider in SDK, atexit moved to MeterProvider

[lzchen]

Addresses [#750]

Introduces a way to start the collection/export process via start_pipeline as part of the MeterProvider.
See below for usage:

metrics.set_meter_provider(MeterProvider())
meter = metrics.get_meter(__name__)
metrics.get_meter_provider().start_pipeline(meter, ConsoleMetricsExporter(), 5)

This makes it so that the user does not have to instantiate a PushController instance themselves (a concept that is quite confusing) and allows the user a mechanism to explicit say when they want to start collecting/exporting metrics. This makes it so that we do not have to worry about any metrics (or views in the future) defined during/after a collection has already started.

As well, the atexit logic initially part of PushController now belongs to the MeterProvider, which will call shutdown on all controllers and exporters upon application exit.

I'm using "pipeline" as the term based off the Go implementation but open to other suggestions.

[aabmass]

LGTM 👍

[toumorokoshi]

Thanks! The API change looks good to me too: legible.

are there any other examples that should be modified as a result of this?

Also one comment on a loose atexit handler in the actual library code.

[ocelotl]

Adding a suggestion for default controllers

[ocelotl]

LGTM

[toumorokoshi]

nitpick, thanks!