scrape: make the initial scrape offset configurable #211

Job	Run time
Build Prometheus for all architectures	0s
Build Prometheus for common architectures (0)	13m 49s
golangci-lint	5m 18s
fuzzing	0s
Build Prometheus for common architectures (1)	13m 48s
Go tests	39m 15s
Build Prometheus for common architectures (2)	13m 47s
Go tests with previous Go version	10m 45s
Mixins tests	3m 19s
UI tests	3m 37s
Go tests on Windows	17m 4s
codeql / Analyze (go)	30m 42s
codeql / Analyze (javascript)	2m 22s
Publish UI on npm Registry	1m 53s
Publish main branch artifacts	0s
Publish release artefacts	0s
	2h 35m 39s

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