fix(relabel): stop randomly dropping labels

[fdegiuli]

applyRelabelConfigs builds its input by ranging over a model.LabelSet, which is a map, so the order is random.
relabel.Process assumes sorted input when it resolves source_labels so on some orderings the lookup misses,
resolves empty, and the rule silently no-ops.

Impact: any rule using source_labels is intermittently skipped, with no error — the target label is just absent.
In our deployment this dropped service_name on ~30% of CloudWatch events.

There's an existing comment in main.go that calls out this requirement, but builder.Sort() was never actually called.

This PR just adds that missing call to Sort()

Added a tiny test that verifies the fix.

Likely also fixes #94 (labeldrop missing some matching labels)

[github-actions]

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1 of 1 commit between main and fix_relabel could not be fully verified:

Commit	Author	Reason	Message
7688b5fd	Federico De Giuli	unknown_key	bugfix: Make relabel application deterministic

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[philiplinell]

I am planning to map CloudWatch log-groups to service labels using relabeling, so this fix would help. Thanks for addressing it!
Hopefully it can be merged and released soon.