Use pod UID as a shard key for "loki.source" clustering

[ptodev]

Request

Alloy has two components for gathering logs which support clustering - loki.source.kubernetes and loki.source.podlogs. In order for those components to divide the work among members of the cluster, they currently hash some of the k8s service discovery labels:

• loki.source.kubernetes uses both public and non-public labels, but since Fix logs cluster #1716 it only takes into account some of them.
• loki.source.podlogs uses a hash of all the "public" labels (the ones which don't start with __).

It would be better if the only label we use is the one whose value is the pod UID. For example, __meta_kubernetes_pod_uid. This is enough to uniquely identify a pod. If there is no such label, and if clustering is enabled, then no logs should be sent.

Use case

This has a few advantages:

• It's safer. A pod UID is guaranteed to identify a pod instance uniquely. There's no possibility of the same pod instance being picked up by several Alloy instances.
• It's more performant. No need to extract all the labels and hash them.
• No possibility of hash collisions.
• Simpler to document and to understand.

The only disadvantage is that it's a breaking change. Some users may need to change their configs.

[ptodev]

I wonder if it's worth using the container ID instead, from a label such as __meta_kubernetes_pod_container_id ?