fix: use stringcontent for metrics and register so we can set headers

[daveleek]

Description

Changes requests to metrics and register in UnleashApiClient, now uses StringContent instead of StreamContent so we can set Content-Length header properly. Makes the SDK play nicer together with Edge

Fixes https://github.com/Unleash/unleash-edge/issues/357

Type of change

• [x] Bug fix (non-breaking change which fixes an issue)

How Has This Been Tested?

Running locally against Edge

Checklist:

• [ ] My code follows the style guidelines of this project
• [ ] I have performed a self-review of my own code
• [ ] I have commented my code, particularly in hard-to-understand areas
• [ ] I have made corresponding changes to the documentation
• [ ] My changes generate no new warnings
• [ ] I have added tests that prove my fix is effective or that my feature works
• [ ] New and existing unit tests pass locally with my changes
• [ ] Any dependent changes have been merged and published in downstream modules

[daveleek]

This has been replaced with #182 as that seems to be the root cause for this issue