log the attribute's key when encoding an attribute fails

[danielhochman]

Description

Include the attribute key that failed to encode. Very hard to track down otherwise.

Before

Invalid type <class 'NoneType'> of value None

After

Failed to encode key namespace: Invalid type <class 'NoneType'> of value None

Type of change

• New feature (non-breaking change which adds functionality)

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

• Unit tests

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

[linux-foundation-easycla]

The committers listed above are authorized under a signed CLA.

• ✅ login: lzchen / name: Leighton Chen (1828093, b3f37b3)
• ✅ login: danielhochman / name: Daniel Hochman (8378caf, 3a9d2c1, de1c4ed, 1667b0a, 8ed207c)
• ✅ login: xrmx / name: Riccardo Magliocchetti (b6f6b48)

[xrmx]

@danielhochman you have to sign the CLA for contributing to OpenTelemetry

[lzchen]

@danielhochman

Please fix the build and add a CHANGELOG entry