Refactor upstream timing capability

[jordanholt]

Description

Add the ability to configure which response header to consider when processing the upstream_duration value and provide a simple Date-based fallback if this is not available.

Since existing v1 consumers may be reliant on the "default" header being used, this would incur a breaking change.

Motivation and Context

In v1, an assumption was made that responses would contain a header with the key x-response-time to be used as the upstream duration. We would like to have the ability to configure this header or alternatively calculate this manually.

How Has This Been Tested?

New unit tests added.

Types of changes

• 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 change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have read the CONTRIBUTING document.
• I have added tests to cover my changes.
• All new and existing tests passed.

[jeanrauwers]

LGTM

[drrobharper]

LGTM 👍