Add readme section on metric label collisions #484

Job	Run time
test-coverage	2m 21s
lint	4m 50s
compatibility-test (1.20, ubuntu-latest, 386)	2m 21s
compatibility-test (1.20, ubuntu-latest, amd64)	2m 23s
compatibility-test (1.20, macos-latest, amd64)	3m 21s
compatibility-test (1.20, windows-latest, 386)	10m 54s
compatibility-test (1.20, windows-latest, amd64)	8m 32s
compatibility-test (1.19, ubuntu-latest, 386)	2m 31s
compatibility-test (1.19, ubuntu-latest, amd64)	2m 20s
compatibility-test (1.19, macos-latest, amd64)	3m 29s
compatibility-test (1.19, windows-latest, 386)	9m 46s
compatibility-test (1.19, windows-latest, amd64)	11m 44s
test-race	2m 53s
	1h 7m 25s

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