feat(prometheus-client-derive): initial implemenation #928

Job	Run time
Clippy	1m 5s
Rustfmt	11s
Test Suite	1m 50s
Cross compile (armv7-unknown-linux-gnueabihf)	15s
Cross compile (powerpc-unknown-linux-gnu)	17s
Cross compile (powerpc64-unknown-linux-gnu)	11s
Cross compile (wasm32-unknown-unknown)	11s
Check	1m 26s
Check rustdoc intra-doc links	24s
	5m 50s

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