Add ServiceSpec for TiKV and Discovery configuration

[wxiaomou]

Feature Request

Currently we can specify ServiceSpec in many components but not tikv services, does it make sense to add ServiceSpec for TiKV and discovery configurations?

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[csuzhangxc]

TiDB service is acting as the endpoint of a TiDB cluster, so we added Service Spec for it. For TiKV and discovery, any details about the scenario?

[wxiaomou]

@csuzhangxc we have internal service (istio) that's inspecting all k8s services, we want to add an custom annotation to tikv and tidb discovery services to let our internal service ignore tikv service.

[csuzhangxc]

As you mentioned, many components have added ServiceSpec, so I think adding it to TiKV for this case is OK.