add support for 0 global attribute limit and tests

[Arnatious]

Description

SpanLimits now properly supports a global attribute limit of 0. Previously it would treat 0 the same as None, and cause a cascade of limits to default.

Added tests to verify this behavior (test_span_zero_global_limit, test_span_zero_global_nonzero_model, test_span_zero_global_unset_model).

Fixes #2392

Type of change

• Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Please describe the tests that you ran to verify your changes. Provide instructions so we can reproduce. Please also list any relevant details for your test configuration

• tox -e py38-opentelemetry-sdk -- trace/test_trace.py::TestSpanLimits

Does This PR Require a Contrib Repo Change?

Answer the following question based on these examples of changes that would require a Contrib Repo Change:

• The OTel specification has changed which prompted this PR to update the method interfaces of opentelemetry-api/ or opentelemetry-sdk/
• The method interfaces of test/util have changed: No
• Scripts in scripts/ that were copied over to the Contrib repo have changed
• Configuration files that were copied over to the Contrib repo have changed (when consistency between repositories is applicable) such as in
  • pyproject.toml
  • isort.cfg
  • .flake8
• When a new .github/CODEOWNER is added: No
• Major changes to project information, such as in:
  • README.md
  • CONTRIBUTING.md
• 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