perf(prometheus.relabel): Reduce lock contention #12329

Job	Run time
fuzz / find-tests	9s
fuzz / syntax/scanner: FuzzSanitizeIdentifier	5m 31s
fuzz / syntax/parser: FuzzParser	5m 34s
fuzz / syntax/scanner: FuzzScanner	5m 33s
fuzz / internal/component/loki/secretfilter: FuzzProcessEntry	7m 5s
fuzz / internal/component/pyroscope/scrape/internal/fastdelta: FuzzDelta	6m 52s
	30m 44s

Provide feedback