add rpc support for getting system metrics #194

Job	Run time
Run linters and tests (Linux-x86_64, ubuntu-latest, x86_64-unknown-linux-musl, coman, coman-Linux...	1m 12s
Run linters and tests (Windows-aarch64, windows-latest, aarch64-pc-windows-msvc, coman.exe, coman...	1m 16s
	2m 28s

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