protobuf: move from OpenMetrics to Prometheus proto #1033

Job	Run time
Test Suite	2m 12s
Clippy	1m 18s
Rustfmt	28s
Cross compile (wasm32-unknown-unknown)	25s
Check	1m 39s
Cross compile (powerpc-unknown-linux-gnu)	11s
Cross compile (powerpc64-unknown-linux-gnu)	35s
Check rustdoc intra-doc links	1m 2s
Cross compile (armv7-unknown-linux-gnueabihf)	26s
	8m 16s

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