Normalize DiskIOMeter bar by number of disks #4498

Job	Run time
build-dragonflybsd-latest-gcc	3m 11s
build-macos-latest-clang	40s
build-freebsd-latest-clang	3m 32s
build-netbsd-latest-gcc	3m 46s
build-openbsd-latest-clang	3m 9s
build-solaris-latest-gcc	5m 17s
build-ubuntu-latest-clang-analyzer	1m 24s
build-ubuntu-latest-clang-sanitizer	1m 3s
build-ubuntu-latest-full-featured-clang	1m 6s
build-ubuntu-latest-full-featured-gcc	50s
build-ubuntu-latest-gcc-static	1m 0s
build-ubuntu-latest-minimal-clang	1m 2s
build-ubuntu-latest-minimal-gcc	40s
build-ubuntu-latest-pcp	26s
lint-pcp-config	3s
lint-whitespace	4s
	27m 13s

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