* Decouple `relays/client-substrate` from `headers_relay`
* Remove `blocks_in_state` from `SyncLoopMetrics`
    This metric was only relevant for PoA <> Substrate bridge.
* Move `sync_loop_metrics.rs` to `relays/finality`
* Remove unused `SyncLoopMetrics::update()`
* Hook up SyncLoopMetrics to finality_loop
* Delete now unused `relays/headers`

Signed-off-by: acatangiu <[email protected]>