Adding api_request_count self-metric when running otlphttp exporter

[rafaelwestphal]

Description

This adds api_request_count when using the otlphttp exporter instead of the GCM/GMP.
Since this uses the HTTP protocol, the previous approach was not compatible.

Related issue

b/481750163

How has this been tested?

Checklist:

• Unit tests
  • Unit tests do not apply.
  • Unit tests have been added/modified and passed for this PR.
• Integration tests
  • Integration tests do not apply.
  • Integration tests have been added/modified and passed for this PR.
• Documentation
  • This PR introduces no user visible changes.
  • This PR introduces user visible changes and the corresponding documentation change has been made.
• Minor version bump
  • This PR introduces no new features.
  • This PR introduces new features, and there is a separate PR to bump the minor version since the last release already.
  • This PR bumps the version.