Add __contains__ to TraceState

[alertedsnake]

Description

Adds __contains__ to TraceState - having this prevents accidentally assuming it's present and will work as expected.

This actually fixes a bug reported in the contrib repo: open-telemetry/opentelemetry-python-contrib#262 and possibly others.

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?

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

• TestTraceContextFormat.test_trace_contains

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

[ocelotl]

This fixes the issue, approving.

That being said, I think we should instead subclass TraceState from OrderedDict so that we don't have to keep adding these methods by hand. If we want to avoid items being set, we can raise a proper exception in __setitem__.

[srikanthccv]

LGTM. Suggested small change.

[alertedsnake]

That broken test seems unrelated, especially since it passed other times, but let me know if there's something I can do.

[ocelotl]

That broken test seems unrelated, especially since it passed other times, but let me know if there's something I can do.

Pypy has this behavior sometimes, I am running the tests in this PR again 👍