Add `latency` to `RelayConnectionStats` #614

Job	Run time
Format	7s
Build WASM (stable, -p nostr)	1m 19s
Build WASM (stable, -p nostr-sdk)	1m 4s
Build WASM (stable, -p nostr-js)	1m 10s
Build WASM (1.64.0, -p nostr)	2m 34s
Build WASM (1.64.0, -p nostr-sdk)	2m 18s
Build WASM (1.64.0, -p nostr-js)	2m 38s
Build (stable, -p nostr)	2m 48s
Build (stable, -p nostr --no-default-features)	1m 29s
Build (stable, -p nostr --no-default-features --features all-nips)	3m 37s
Build (stable, -p nostr --no-default-features --features vanity)	1m 35s
Build (stable, -p nostr --features blocking)	2m 28s
Build (stable, -p nostr-sdk)	2m 51s
Build (stable, -p nostr-sdk --no-default-features)	1m 49s
Build (stable, -p nostr-sdk --features blocking)	2m 37s
Build (stable, -p nostr-ffi)	3m 18s
Build (stable, -p nostr-sdk-ffi)	3m 38s
Build (1.64.0, -p nostr)	4m 26s
Build (1.64.0, -p nostr --no-default-features)	4m 17s
Build (1.64.0, -p nostr --no-default-features --features all-nips)	6m 8s
Build (1.64.0, -p nostr --no-default-features --features vanity)	3m 22s
Build (1.64.0, -p nostr --features blocking)	6m 30s
Build (1.64.0, -p nostr-sdk)	5m 15s
Build (1.64.0, -p nostr-sdk --no-default-features)	3m 33s
Build (1.64.0, -p nostr-sdk --features blocking)	4m 43s
Build (1.64.0, -p nostr-ffi)	4m 59s
Build (1.64.0, -p nostr-sdk-ffi)	6m 16s
	1h 26m 49s

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