Optimize zlabel usage for stringlabels #15886

Job	Run time
Linters (Static Analysis) for Go	4m 26s
Go build for different platforms	10m 12s
Check misspelled words	15s
Thanos end-to-end tests (8, 0)	5m 34s
Thanos unit tests	12m 36s
Thanos end-to-end tests (8, 1)	4m 36s
Go build with -tags=stringlabels	1m 59s
Thanos end-to-end tests (8, 2)	6m 4s
Thanos end-to-end tests (8, 3)	5m 28s
Thanos end-to-end tests (8, 4)	7m 59s
Thanos end-to-end tests (8, 5)	5m 21s
Thanos end-to-end tests (8, 6)	5m 5s
Thanos end-to-end tests (8, 7)	4m 42s
	1h 14m 17s

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