test(metrics): replace random value with a static value

[u-kai]

What does it do ?

This PR replaces the randomly generated Subsystem names in internal/gen/docs/metrics/main_test.go with static, unique values.

Previously, rand.IntN(100) was used to reduce the chance of collision, but this still introduced a 1% chance of flaky test failures due to duplicate names.

Motivation

In #5459, we discussed improving test stability by adding +100 to the second rand.IntN call.
While that suggestion would reduce the collision probability, I opted for static values instead.
This eliminates the chance of collision entirely and ensures that if a conflicting test is added in the future, it will fail deterministically and be immediately detectable during development or CI.

More

• Yes, this PR title follows Conventional Commits
• Yes, I added unit tests
• Yes, I updated end user documentation accordingly

[k8s-ci-robot]

Hi @u-kai. Thanks for your PR.

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[mloiseleur]

/ok-to-test

[ivankatliarchuk]

/lgtm

[mloiseleur]

/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: ivankatliarchuk, mloiseleur

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