Add fields method

[ocelotl]

Fixes #224

Description

Please include a summary of the change and which issue is fixed. Please also include relevant motivation and context. List any dependencies that are required for this change.

Fixes # (issue)

Type of change

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• 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

• Test A

Does This PR Require a Core Repo Change?

• Yes. - Link to PR:
• No.

Checklist:

See contributing.md for styleguide, changelog guidelines, and more.

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

[codeboten]

Does this need an entry in the changelog for the aws sdk?

[ocelotl]

Does this need an entry in the changelog for the aws sdk?

Added this changelog entry.

[codeboten]

LGTM

[NathanielRN]

Nice! Thank you 😄

Also, maybe a test that compares the fields returned by fields() and the fields set after an inject would be helpful?

[NathanielRN]

Here is the Core repo PR: open-telemetry/opentelemetry-python#1374