cumulus/minimal-node: added prometheus metrics for the RPC client #1108

Job	Run time
preflight / preflight	36s
cargo-check-benches (master)	23m 30s
cargo-check-benches (current)	23m 31s
cargo-check-each-crate (1)	15m 20s
cargo-check-each-crate (2)	18m 2s
cargo-check-each-crate (3)	4m 58s
cargo-check-each-crate (4)	13m 42s
test-full-crypto-feature	2m 35s
test-frame-ui	23m 26s
cargo-check-each-crate (5)	22m 38s
test-node-metrics	8m 22s
cargo-check-each-crate (6)	13m 24s
cargo-check-each-crate (7)	23m 29s
cargo-check-all-crate-macos	10m 53s
test-frame-examples-compile-to-wasm	2m 56s
test-deterministic-wasm	6m 26s
check-tracing	2m 18s
check-metadata-hash	5m 52s
cargo-hfuzz	6m 3s
node-bench-regression-guard	1s
All test misc tests passed	1s
	3h 48m 3s

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