Add PeriodicExportingMetricReader usage example to MeterProvider docstring

Description

The MeterProvider docstring in opentelemetry-sdk/src/opentelemetry/sdk/metrics/_internal/__init__.py had a TODO comment in the metric_readers argument description:

TODO: reference PeriodicExportingMetricReader usage with push exporters here.

This PR resolves the TODO by:

1. Replacing the TODO with a :class: cross reference to opentelemetry.sdk.metrics.export.PeriodicExportingMetricReader in
   the metric_readers arg description
2. Adding a .. code-block:: python example demonstrating push based export setup using PeriodicExportingMetricReader and
   ConsoleMetricExporter, consistent in style with the existing Disable default views example in the same docstring

Type of change

• This change requires a documentation update

How Has This Been Tested?

• Ran tox -e lint ✅
• Ran tox -e py ✅

Does This PR Require a Contrib Repo Change?

• No.

Checklist:

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated