feat(xanthous/server): Add simple prometheus metrics

Add a prometheus exporter and some simple prometheus metrics, so that I
can look at dashboards and get alerts for things like lots of
connections

Change-Id: Ic1e0568200299dc852b74da647a6354267ee7576
Reviewed-on: https://cl.tvl.fyi/c/depot/+/3811
Reviewed-by: grfn <grfn@gws.fyi>
Tested-by: BuildkiteCI

users/grfn/xanthous/server/src/metrics.rs

@@ -0,0 +1,24 @@
+pub use ::metrics::*;
+
+pub mod reported {
+    /// Counter: Connections accepted on the TCP listener
+    pub const CONNECTIONS_ACCEPTED: &str = "ssh.connections.accepted";
+
+    /// Histogram: Connection duration
+    pub const CONNECTION_DURATION: &str = "ssh.connections.duration";
+
+    /// Gauge: Currently active connections
+    pub const ACTIVE_CONNECTIONS: &str = "ssh.connections.active";
+
+    /// Gauge: Currently running xanthous processes
+    pub const RUNNING_PROCESSES: &str = "ssh.child.processes";
+}
+
+pub fn register() {
+    use reported::*;
+
+    register_counter!(CONNECTIONS_ACCEPTED);
+    register_histogram!(CONNECTION_DURATION);
+    register_gauge!(ACTIVE_CONNECTIONS);
+    register_gauge!(RUNNING_PROCESSES);
+}