[release/3][BACKPORT] Prometheus: Enable etcd rules

[gracedo]

This is a backport of the following PR:

#938

What type of PR is this?

bug

What this PR does/ why we need it:

When I removed mesosphere-specific etcd rules to use the default, I didn't realize the config here needed to be updated to reflect those changes. I tested adding the rule to the daily cluster and verified all the rules look good.

The change was made in v9.3.4, so we need to backport this to konvoy 1.7/1.6/1.5 (release/3.3, release/3.0, release/2.5)

Also, removed some unnecessary configs that have since been added to the chart values as default (mesosphere/charts#969, https://github.com/mesosphere/charts/pull/962/files#diff-4eefbe60d75fc0250847e2e4dfd675f9977d572e6f0b5615b552b8223719a6caR2324)

Which issue(s) this PR fixes:

Special notes for your reviewer:

Does this PR introduce a user-facing change?:

prometheus(fix): Re-enable etcd prometheus rules

Checklist

• The commit message explains the changes and why are needed.
• The code builds and passes lint/style checks locally.
• The relevant subset of integration tests pass locally.
• The core changes are covered by tests.
• The documentation is updated where needed.

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