Fixing deprecated metric_start_time processor name

[rafaelwestphal]

Description

The metric_start_time processor changed name to match new convention
open-telemetry/opentelemetry-collector-contrib#46777

Related issue

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.

[LujieDuan]

LGTM