Query: Fix panic in ZLabelsToPromLabels due to unsafe pointer cast incompatible with stringlabels

[coleenquadros]

• The old implementation reinterpreted memory between []ZLabel (slice of structs) and labels.Labels via unsafe.Pointer, which broke when Prometheus v0.308.0 changed labels.Labels from a slice-of-structs to a compact string encoding (stringlabels)
• This caused a panic in the Query component during endpoint discovery: panic: runtime error: index out of range [108] with length 1 at labels_stringlabels.go:decodeSize

• I added CHANGELOG entry for this change.
• Change is not relevant to the end user.

Changes

Verification

[GiedriusS]

It's a bit more involved. For starters, you can read https://giedrius.blog/2022/02/04/things-learned-from-trying-to-migrate-to-protobuf-v2-api-from-gogoprotobuf-so-far/ or look through my previous PRs on this repo. I personally think we need to handwrite the marshaling/unmarshaling functions that construct labels.Labels directly, and then return the used memory to a pool on query.Close(). Feel free to jump onto the CNCF slack to discuss further.

[coleenquadros]

@GiedriusS however is this failure/panic specific to a set up? I was using v0.41.0 version and the query, query frontend pods were crashlooping on bring up