Make requests span attributes available to samplers

[trask]

Description

Updates requests instrumentation based on http semantic conventions:

Following attributes MUST be provided at span creation time (when provided at all), so they can be considered for sampling decisions:

• http.method
• http.url
• ...

Type of change

Please delete options that are not relevant.

• 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

• requests instrumentation tests were run

Does This PR Require a Core Repo Change?

• No.

Checklist:

See contributing.md for styleguide, changelog guidelines, and more.

• Followed the style guidelines of this project
• Changelogs have been updated
• Unit tests have been added
• Documentation has been updated