Normalize DiskIOMeter bar by number of disks #4569

Job	Run time
build-ubuntu-latest-full-featured-gcc	48s
build-ubuntu-latest-minimal-clang	58s
build-ubuntu-latest-full-featured-clang	1m 22s
build-dragonflybsd-latest-gcc	2m 17s
build-macos-latest-clang	46s
build-freebsd-latest-clang	2m 26s
build-netbsd-latest-gcc	2m 43s
build-ubuntu-latest-clang-analyzer	1m 14s
build-ubuntu-latest-clang-sanitizer	1m 6s
build-ubuntu-latest-minimal-gcc	39s
build-ubuntu-latest-gcc-static	53s
build-ubuntu-latest-pcp	22s
build-openbsd-latest-clang	2m 22s
build-solaris-latest-gcc	3m 27s
lint-pcp-config	2s
lint-whitespace	3s
	21m 28s

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