Add prometheus protobuf #5

Job	Run time
Test Suite	1m 58s
Check rustdoc intra-doc links	30s
Cross compile (powerpc-unknown-linux-gnu)	19s
Check	1m 41s
Cross compile (wasm32-unknown-unknown)	22s
Cross compile (armv7-unknown-linux-gnueabihf)	18s
Rustfmt	23s
Clippy	1m 12s
Cross compile (powerpc64-unknown-linux-gnu)	19s
	7m 2s

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