protobuf: move from OpenMetrics to Prometheus proto (#298) #1037

Job	Run time
Check rustdoc intra-doc links	33s
Cross compile (powerpc64-unknown-linux-gnu)	14s
Test Suite	1m 59s
Cross compile (armv7-unknown-linux-gnueabihf)	19s
Cross compile (powerpc-unknown-linux-gnu)	12s
Cross compile (wasm32-unknown-unknown)	27s
Check	1m 38s
Rustfmt	19s
Clippy	1m 10s
	6m 51s

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