Move HTTP client metrics out of .NET conventions

[joaopgrassi]

Fixes #800

Changes

This PR moves HTTP client metrics (and one attribute) from the .NET metrics model to the general HTTP one.

It removes MD generation from .NET metric definition as it's frozen in time and should not get any updates or changes on the common metric.

It helps to untie .NET semconv stability from the unstable OTel parts it depends upon and helps with #781

Related to #785

Merge requirement checklist

• CONTRIBUTING.md guidelines followed.
• Change log entry added, according to the guidelines in When to add a changelog entry.
  • If your PR does not need a change log, start the PR title with [chore]
• schema-next.yaml updated with changes to existing conventions.

[lmolkova]

@antonfirsov could you please take another look?

[lmolkova]

also //cc @open-telemetry/dotnet-approvers @open-telemetry/semconv-dotnet-approver