Refactor HttpClient operation-id header

[rodolfoams]

What is the purpose of this pull request?

Tiny refactor of the HttpClient operation id header definition.

What problem is this solving?

Remove inconsistency with header addition.

How should this be manually tested?

Screenshots or example usage

Types of changes

• [ ] Bug fix (a non-breaking change which fixes an issue)
• [ ] New feature (a non-breaking change which adds functionality)
• [ ] Breaking change (fix or feature that would cause existing functionality to change)
• [ ] Requires change to documentation, which has been updated accordingly.