Normalize DiskIOMeter bar by number of disks #4513

Job	Run time
build-macos-latest-clang	45s
build-dragonflybsd-latest-gcc	2m 47s
build-netbsd-latest-gcc	3m 28s
build-freebsd-latest-clang	2m 35s
build-openbsd-latest-clang	2m 56s
build-solaris-latest-gcc	4m 18s
build-ubuntu-latest-clang-analyzer	1m 9s
build-ubuntu-latest-clang-sanitizer	1m 4s
build-ubuntu-latest-full-featured-clang	1m 2s
build-ubuntu-latest-full-featured-gcc	46s
build-ubuntu-latest-gcc-static	51s
build-ubuntu-latest-minimal-clang	1m 1s
build-ubuntu-latest-minimal-gcc	37s
build-ubuntu-latest-pcp	23s
lint-pcp-config	3s
lint-whitespace	2s
	23m 47s

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