Change metric.Producer to be an Option on Reader #8945

Job	Run time
compatibility-test (1.20, ubuntu-latest, 386)	1m 26s
compatibility-test (1.20, ubuntu-latest, amd64)	1m 33s
compatibility-test (1.20, macos-latest, amd64)	1m 24s
compatibility-test (1.20, windows-latest, 386)	6m 17s
compatibility-test (1.20, windows-latest, amd64)	7m 18s
compatibility-test (1.19, ubuntu-latest, 386)	1m 28s
compatibility-test (1.19, ubuntu-latest, amd64)	49s
compatibility-test (1.19, macos-latest, amd64)	11m 41s
compatibility-test (1.19, windows-latest, 386)	5m 0s
compatibility-test (1.19, windows-latest, amd64)	5m 35s
lint	3m 57s
test-coverage	1m 50s
test-race	1m 18s
test-compatibility	1s
	49m 37s

Provide feedback