protobuf: move from OpenMetrics to Prometheus proto #1027

Job	Run time
Check	1m 40s
Clippy	1m 16s
Test Suite	1m 49s
Cross compile (wasm32-unknown-unknown)	19s
Cross compile (armv7-unknown-linux-gnueabihf)	19s
Cross compile (powerpc-unknown-linux-gnu)	20s
Cross compile (powerpc64-unknown-linux-gnu)	19s
Rustfmt	24s
Check rustdoc intra-doc links	31s
	6m 57s

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