feat(prometheus-client-derive): initial implemenation #923

Job	Run time
Test Suite	1m 47s
Cross compile (armv7-unknown-linux-gnueabihf)	13s
Rustfmt	10s
Cross compile (powerpc-unknown-linux-gnu)	13s
Clippy	43s
Cross compile (powerpc64-unknown-linux-gnu)	10s
Cross compile (wasm32-unknown-unknown)	10s
Check	1m 43s
Check rustdoc intra-doc links	20s
	5m 29s

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