pkg/query: Use hash of labels instead of marshaling labels to json

[brancz]

Profiling data showed that:

• building the sample label map
• string interactions involved with dealing with keys/values
• map accesses

Were quite expensive (10-15% CPU of flamegraph rendering).

xxhash3 is extremely fast (it's in a way replacing the json being built), and we'll no longer use strings for key/values anywhere, which is much faster to begin with. This does come with the veeeeeery small chance of a hash collision of label-sets. I'd happily see that happen though and then address it.

[alwaysmeticulous]

✅ Meticulous spotted zero visual differences across 285 screens tested: view results.

_{Last updated for commit b98d6db. This comment will update as new commits are pushed.}

[metalmatze]

Given this is only on the first level of the flame graph the chances for collisions seem indeed very low.