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Add opt-in Ractor-local GC metrics to the Ractor harness #2094

Job	Run time
benchmark-run-once	1m 22s
test (truffleruby)	1m 11s
benchmark-ractor-only (head)	26s
benchmark-default (truffleruby, --excludes=knucleotide,lobsters,ruby-lsp,shipit)	8m 58s
benchmark-ractor (head)	6m 33s
test (ruby)	1m 3s
benchmark-graph	47s
benchmark-default (ruby)	8m 48s
test (head)	48s
benchmark-default (head)	13m 35s
all-tests	2s
	43m 33s