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Add opt-in Ractor-local GC metrics to the Ractor harness #2083

Job	Run time
benchmark-ractor-only (head)	34s
benchmark-run-once	1m 2s
test (head)	2m 46s
benchmark-ractor (head)	5m 30s
benchmark-default (ruby)	12m 42s
benchmark-default (truffleruby, --excludes=knucleotide,lobsters,ruby-lsp,shipit)	10m 19s
benchmark-default (head)	13m 50s
test (ruby)	1m 10s
benchmark-graph	2m 20s
test (truffleruby)	1m 48s
all-tests	3s
	52m 4s