Replace metrics batch processor with sending queue

[rafaelwestphal]

Description

This adds a sending_queue to the metric otlp_grpc metrics exporter.

Related issue

b/485613604

How has this been tested?

Integration tests.

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.

[XuechunHou]

We may want to increase the flush_timeout

https://screenshot.googleplex.com/LtnCgHzkACvqTHW Cloud Monitoring requires data points for a time series to be sent at least five seconds apart. We could set the flush timeout to 5 seconds.