HTTP Client lib: make request label option mandatory

[eduard-fa]

Changes

Based on our internal discussion, we make requestLabel mandatory to ensure it is always present in our requests.

Checklist

• Apply one of following labels; major, minor, patch or skip-release
• I've updated the documentation, or no changes were necessary
• I've updated the tests, or no changes were necessary

[kibertoad]

It's a semver major.

[eduard-fa]

It's a semver major.

While it is a breaking change, I'm slightly hesitant to do a major version bump just because of that tiny change. Is it worth it? Could we postpone releasing it until we accrue a few more changes?

[kibertoad]

Numbers are cheap. Considering that we are not maintaining a migration guide, I would argue that smaller major releases are likely to be way easier to migrate to.
No strong opinion, though. If you prefer to defer, we can mark as skip-release.

[kibertoad]

Let's release this together with #90