feat(prometheus-client-derive): initial implemenation #926

Job	Run time
Rustfmt	8s
Clippy	1m 20s
Test Suite	1m 51s
Check	1m 44s
Check rustdoc intra-doc links	21s
Cross compile (armv7-unknown-linux-gnueabihf)	11s
Cross compile (powerpc-unknown-linux-gnu)	12s
Cross compile (powerpc64-unknown-linux-gnu)	20s
Cross compile (wasm32-unknown-unknown)	10s
	6m 17s

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