add hook to allow head compaction to create multiple output blocks #48

Job	Run time
Go tests	16m 37s
UI tests	2m 25s
Go tests with previous Go version	7m 20s
Build Prometheus for common architectures (0)	6m 51s
Build Prometheus for common architectures (1)	6m 49s
Build Prometheus for all architectures	0s
Build Prometheus for common architectures (2)	6m 26s
Check generated parser	11s
fuzzing	0s
golangci-lint	32s
Mixins tests	2m 9s
More Go tests	9m 23s
Go tests on Windows	12m 28s
codeql / Analyze (javascript)	1m 59s
Publish UI on npm Registry	1m 21s
Publish release artefacts	0s
Publish main branch artifacts	0s
	1h 14m 31s

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