`distributor.metric-relabeling-enabled` can currupt tsdb blocks

[Logiraptor]

Describe the bug

When distributor.metric-relabeling-enabled is true, we have observed that some blocks will fail to compact with the error:

block id 01HP0B0R7BPAWCQDJR74CJFA17: index contains 3 postings with out of order labels

This should not be possible.

To Reproduce

Steps to reproduce the behavior:

1. Start Mimir with metric-relabeling disabled
2. Wait for compactor to attempt compacting some blocks

Expected behavior

Blocks should not be currupt

Additional Context

Current theory is that this line is actually critical for the write path correctness:

mimir/pkg/distributor/distributor.go

Line 831 in 1af7e13

req.Timeseries[tsIdx].SortLabelsIfNeeded()