`MemPostings.PostingsForLabelMatching()`: don't hold the mutex while … #27

Job	Run time
Build Prometheus for all architectures	0s
Build Prometheus for common architectures (0)	55s
Check generated parser	12s
fuzzing	0s
Build Prometheus for common architectures (1)	1m 12s
golangci-lint	2m 52s
Go tests	16m 31s
More Go tests	9m 18s
Go tests with previous Go version	7m 17s
Build Prometheus for common architectures (2)	1m 11s
Mixins tests	2m 18s
UI tests	2m 41s
Go tests on Windows	10m 10s
codeql / Analyze (javascript)	1m 45s
Publish UI on npm Registry	1m 27s
Publish release artefacts	0s
Publish main branch artifacts	0s
	57m 49s

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