fix: Add missing metric-gateway-priority-class argument

[shorim]

Description

Changes proposed in this pull request (what was done and why):

• Add the missing metric-gateway-priority-class argument when running the manager container. In the PR for including the metrics feature in releases, we removed the Kustomize patch for the manager deployment, as it was not needed anymore, since the metrics feature will be enabled by default. However, it was forgotten to copy over the argument for the metric-gateway-priority-class to the manager deployment.

Changes refer to particular issues, PRs or documents:

• Have the metrics features as part of the regular release #623

Traceability

• The PR is linked to a GitHub issue.
• New features have a milestone set.
• New features have defined acceptance criteria in a corresponding GitHub Issue, and all criteria are satisfied with this PR.
• The corresponding GitHub issue has a respective area and kind label.
• The follow-up issues (if any) are linked in the Related Issues section.
• Adjusted the documentation if the change is user-facing.
• The feature is unit-tested
• The feature is e2e-tested