swarm: Split off "keep alive" functionality from DummyConnectionHandler (#2859)

Previously, the `DummyConnectionHandler` offered a "keep alive" functionality,
i.e. it allowed users to set the value of what is returned from
`ConnectionHandler::keep_alive`. This handler is primarily used in tests or
`NetworkBehaviour`s that don't open any connections (like mDNS). In all of these
cases, it is statically known whether we want to keep connections alive. As
such, this functionality is better represented by a static
`KeepAliveConnectionHandler` that always returns `KeepAlive::Yes` and a
`DummyConnectionHandler` that always returns `KeepAlive::No`.

To follow the naming conventions described in
https://github.com/libp2p/rust-libp2p/issues/2217, we introduce a top-level
`keep_alive` and `dummy` behaviour in `libp2p-swarm` that contains both the
`NetworkBehaviour` and `ConnectionHandler` implementation for either case.

misc/metrics/examples/metrics/main.rs

@@ -48,19 +48,19 @@
 //! You should see a long list of metrics printed to the terminal. Check the
 //! `libp2p_ping` metrics, they should be `>0`.
 
+use env_logger::Env;
 use futures::executor::block_on;
 use futures::stream::StreamExt;
 use libp2p::core::Multiaddr;
 use libp2p::metrics::{Metrics, Recorder};
-use libp2p::ping;
 use libp2p::swarm::SwarmEvent;
-use libp2p::{identity, PeerId, Swarm};
+use libp2p::{identity, ping, NetworkBehaviour, PeerId, Swarm};
+use libp2p_swarm::keep_alive;
+use log::info;
 use prometheus_client::registry::Registry;
 use std::error::Error;
 use std::thread;
 
-use env_logger::Env;
-use log::info;
 mod http_service;
 
 fn main() -> Result<(), Box<dyn Error>> {
@@ -72,7 +72,7 @@ fn main() -> Result<(), Box<dyn Error>> {
 
     let mut swarm = Swarm::new(
         block_on(libp2p::development_transport(local_key))?,
-        ping::Behaviour::new(ping::Config::new().with_keep_alive(true)),
+        Behaviour::default(),
         local_peer_id,
     );
 
@@ -91,7 +91,7 @@ fn main() -> Result<(), Box<dyn Error>> {
     block_on(async {
         loop {
             match swarm.select_next_some().await {
-                SwarmEvent::Behaviour(ping_event) => {
+                SwarmEvent::Behaviour(BehaviourEvent::Ping(ping_event)) => {
                     info!("{:?}", ping_event);
                     metrics.record(&ping_event);
                 }
@@ -104,3 +104,13 @@ fn main() -> Result<(), Box<dyn Error>> {
     });
     Ok(())
 }
+
+/// Our network behaviour.
+///
+/// For illustrative purposes, this includes the [`keep_alive::Behaviour`]) behaviour so the ping actually happen
+/// and can be observed via the metrics.
+#[derive(NetworkBehaviour, Default)]
+struct Behaviour {
+    keep_alive: keep_alive::Behaviour,
+    ping: ping::Behaviour,
+}