Fix simple race condition in traffic metric/plugin E2E test

[jgwest]

What does this PR do / why we need it:

• Fix simple race condition in traffic metric/plugin E2E test

Failing case looks like this within Argo Rollouts Pod:

[jgw@localhost-lan argo-rollouts-manager]$ k logs pod/argo-rollouts-c854d44bc-ftlss
time="2025-07-31T02:43:31Z" level=info msg="Argo Rollouts starting" version=vv1.8.3+49fa151
time="2025-07-31T02:43:31Z" level=info msg="Creating event broadcaster"
time="2025-07-31T02:43:31Z" level=info msg="Setting up event handlers"
time="2025-07-31T02:43:31Z" level=info msg="Setting up experiments event handlers"
time="2025-07-31T02:43:31Z" level=info msg="Setting up analysis event handlers"
time="2025-07-31T02:43:31Z" level=info msg="Downloading plugin argoproj-labs/gatewayAPI from: https://test-update-traffic-plugin"
time="2025-07-31T02:43:31Z" level=fatal msg="Failed to download plugins: failed to download plugin from https://test-update-traffic-plugin: failed to download file from https://test-update-traffic-plugin: Get \"https://test-update-traffic-plugin\": dial tcp: lookup test-update-traffic-plugin on 172.30.0.10:53: no such host"

Have you updated the necessary documentation?

• Documentation update is required by this PR, and has been updated.

How to test changes / Special notes to the reviewer:

[jparsai]

LGTM

[svghadi]

/lgtm
/approve