feat(prometheus-client-derive): initial implemenation #924

Job	Run time
Rustfmt	10s
Clippy	1m 8s
Check	1m 31s
Check rustdoc intra-doc links	19s
Cross compile (armv7-unknown-linux-gnueabihf)	11s
Test Suite	1m 47s
Cross compile (powerpc-unknown-linux-gnu)	11s
Cross compile (powerpc64-unknown-linux-gnu)	28s
Cross compile (wasm32-unknown-unknown)	18s
	6m 3s

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