test: Validate Istio metric attributes

[chrkl]

Description

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

• PR fix: Do not intercept outgoing connections from metric agent by Istio #609 fixed a but to not intercept the metric scraping of Istio proxies, which was indicated by wrong metric attributes. This PR enhances the corresponding integration test to validate that Istio metrics have the right attribute values.

Changes refer to particular issues, PRs or documents:

• Istio metrics payload has content of agents sidecar #606

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