Remove unused relays/headers (#1216)

* 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 <adrian@parity.io>

bridges/relays/finality/src/sync_loop_metrics.rs

@@ -0,0 +1,60 @@
+// Copyright 2019-2021 Parity Technologies (UK) Ltd.
+// This file is part of Parity Bridges Common.
+
+// Parity Bridges Common is free software: you can redistribute it and/or modify
+// it under the terms of the GNU General Public License as published by
+// the Free Software Foundation, either version 3 of the License, or
+// (at your option) any later version.
+
+// Parity Bridges Common is distributed in the hope that it will be useful,
+// but WITHOUT ANY WARRANTY; without even the implied warranty of
+// MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+// GNU General Public License for more details.
+
+// You should have received a copy of the GNU General Public License
+// along with Parity Bridges Common.  If not, see <http://www.gnu.org/licenses/>.
+
+//! Metrics for headers synchronization relay loop.
+
+use relay_utils::metrics::{metric_name, register, GaugeVec, Opts, PrometheusError, Registry, U64};
+
+/// Headers sync metrics.
+#[derive(Clone)]
+pub struct SyncLoopMetrics {
+	/// Best syncing headers at "source" and "target" nodes.
+	best_block_numbers: GaugeVec<U64>,
+}
+
+impl SyncLoopMetrics {
+	/// Create and register headers loop metrics.
+	pub fn new(registry: &Registry, prefix: Option<&str>) -> Result<Self, PrometheusError> {
+		Ok(SyncLoopMetrics {
+			best_block_numbers: register(
+				GaugeVec::new(
+					Opts::new(
+						metric_name(prefix, "best_block_numbers"),
+						"Best block numbers on source and target nodes",
+					),
+					&["node"],
+				)?,
+				registry,
+			)?,
+		})
+	}
+}
+
+impl SyncLoopMetrics {
+	/// Update best block number at source.
+	pub fn update_best_block_at_source<Number: Into<u64>>(&self, source_best_number: Number) {
+		self.best_block_numbers
+			.with_label_values(&["source"])
+			.set(source_best_number.into());
+	}
+
+	/// Update best block number at target.
+	pub fn update_best_block_at_target<Number: Into<u64>>(&self, target_best_number: Number) {
+		self.best_block_numbers
+			.with_label_values(&["target"])
+			.set(target_best_number.into());
+	}
+}