protocols/: Implement Direct Connection Upgrade through Relay (DCUtR) (#2438)

Enables two peers to coordinate a hole punch (direct connection upgrade)
via a relayed connection.

See https://github.com/libp2p/specs/blob/master/relay/DCUtR.md for
specification.

misc/metrics/src/dcutr.rs

@@ -0,0 +1,89 @@
+// Copyright 2021 Protocol Labs.
+//
+// Permission is hereby granted, free of charge, to any person obtaining a
+// copy of this software and associated documentation files (the "Software"),
+// to deal in the Software without restriction, including without limitation
+// the rights to use, copy, modify, merge, publish, distribute, sublicense,
+// and/or sell copies of the Software, and to permit persons to whom the
+// Software is furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in
+// all copies or substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS
+// OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+// FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+// AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+// LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING
+// FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+// DEALINGS IN THE SOFTWARE.
+
+use prometheus_client::encoding::text::Encode;
+use prometheus_client::metrics::counter::Counter;
+use prometheus_client::metrics::family::Family;
+use prometheus_client::registry::Registry;
+
+pub struct Metrics {
+    events: Family<EventLabels, Counter>,
+}
+
+impl Metrics {
+    pub fn new(registry: &mut Registry) -> Self {
+        let sub_registry = registry.sub_registry_with_prefix("relay");
+
+        let events = Family::default();
+        sub_registry.register(
+            "events",
+            "Events emitted by the relay NetworkBehaviour",
+            Box::new(events.clone()),
+        );
+
+        Self { events }
+    }
+}
+
+#[derive(Debug, Clone, Hash, PartialEq, Eq, Encode)]
+struct EventLabels {
+    event: EventType,
+}
+
+#[derive(Debug, Clone, Hash, PartialEq, Eq, Encode)]
+enum EventType {
+    InitiateDirectConnectionUpgrade,
+    RemoteInitiatedDirectConnectionUpgrade,
+    DirectConnectionUpgradeSucceeded,
+    DirectConnectionUpgradeFailed,
+}
+
+impl From<&libp2p_dcutr::behaviour::Event> for EventType {
+    fn from(event: &libp2p_dcutr::behaviour::Event) -> Self {
+        match event {
+            libp2p_dcutr::behaviour::Event::InitiatedDirectConnectionUpgrade {
+                remote_peer_id: _,
+                local_relayed_addr: _,
+            } => EventType::InitiateDirectConnectionUpgrade,
+            libp2p_dcutr::behaviour::Event::RemoteInitiatedDirectConnectionUpgrade {
+                remote_peer_id: _,
+                remote_relayed_addr: _,
+            } => EventType::RemoteInitiatedDirectConnectionUpgrade,
+            libp2p_dcutr::behaviour::Event::DirectConnectionUpgradeSucceeded {
+                remote_peer_id: _,
+            } => EventType::DirectConnectionUpgradeSucceeded,
+            libp2p_dcutr::behaviour::Event::DirectConnectionUpgradeFailed {
+                remote_peer_id: _,
+                error: _,
+            } => EventType::DirectConnectionUpgradeFailed,
+        }
+    }
+}
+
+impl super::Recorder<libp2p_dcutr::behaviour::Event> for super::Metrics {
+    fn record(&self, event: &libp2p_dcutr::behaviour::Event) {
+        self.dcutr
+            .events
+            .get_or_create(&EventLabels {
+                event: event.into(),
+            })
+            .inc();
+    }
+}