fix: clear unwanted global variables in module init

[gen-xu]

Description

I found this by trying to use importlib.reload.

import importlib
import opentelemetry.sdk.metrics as metrics
importlib.reload(metrics)

will raise error like this

Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/gen/.pyenv/versions/3.11.1/lib/python3.11/importlib/__init__.py", line 169, in reload
    _bootstrap._exec(spec, module)
  File "<frozen importlib._bootstrap>", line 621, in _exec
  File "<frozen importlib._bootstrap_external>", line 940, in exec_module
  File "<frozen importlib._bootstrap>", line 241, in _call_with_frames_removed
  File "/Users/gen/.pyenv/versions/3.11.1/lib/python3.11/site-packages/opentelemetry/sdk/metrics/__init__.py", line 36, in <module>
    value.__module__ = __name__
    ^^^^^^^^^^^^^^^^
AttributeError: 'str' object has no attribute '__module__'

The str above is basically the key in the previous run of this loop.

Though I understand that reload isn't any guaranteed behavior, but without del key, value it polluted the module level globals, that I think is unexected.

Type of change

Please delete options that are not relevant.

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• This change requires a documentation update

How Has This Been Tested?

import importlib
import opentelemetry.sdk.metrics as metrics
importlib.reload(metrics)

Does This PR Require a Contrib Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

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

[shalevr]

Is it possible to use private variables instead of deleting them?
Like this:
for __key, __value in globals().copy().items():

[ocelotl]

We have reverted to a static to define __all__ (see #3143), thanks for this PR, though 🙂