protobuf: move from OpenMetrics to Prometheus proto #1032

Job	Run time
Check	1m 59s
Check rustdoc intra-doc links	41s
Clippy	1m 18s
Rustfmt	22s
Cross compile (powerpc-unknown-linux-gnu)	22s
Cross compile (armv7-unknown-linux-gnueabihf)	20s
Cross compile (wasm32-unknown-unknown)	18s
Cross compile (powerpc64-unknown-linux-gnu)	13s
Test Suite	1m 51s
	7m 24s

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