proxy: expose session_id to clients and proxy latency to probes (#12656)

Implements #8728

proxy/src/binary/proxy.rs

@@ -1,4 +1,3 @@
-#[cfg(any(test, feature = "testing"))]
 use std::env;
 use std::net::SocketAddr;
 use std::path::PathBuf;
@@ -21,7 +20,7 @@ use tokio::net::TcpListener;
 use tokio::sync::Notify;
 use tokio::task::JoinSet;
 use tokio_util::sync::CancellationToken;
-use tracing::{error, info, warn};
+use tracing::{debug, error, info, warn};
 use utils::sentry_init::init_sentry;
 use utils::{project_build_tag, project_git_version};
 
@@ -730,6 +729,25 @@ fn build_config(args: &ProxyCliArgs) -> anyhow::Result<&'static ProxyConfig> {
         }
     };
 
+    let mut greetings = env::var_os("NEON_MOTD").map_or(String::new(), |s| match s.into_string() {
+        Ok(s) => s,
+        Err(_) => {
+            debug!("NEON_MOTD environment variable is not valid UTF-8");
+            String::new()
+        }
+    });
+
+    match &args.auth_backend {
+        AuthBackendType::ControlPlane => {}
+        #[cfg(any(test, feature = "testing"))]
+        AuthBackendType::Postgres => {}
+        #[cfg(any(test, feature = "testing"))]
+        AuthBackendType::Local => {}
+        AuthBackendType::ConsoleRedirect => {
+            greetings = "Connected to database".to_string();
+        }
+    }
+
     let config = ProxyConfig {
         tls_config,
         metric_collection,
@@ -740,6 +758,7 @@ fn build_config(args: &ProxyCliArgs) -> anyhow::Result<&'static ProxyConfig> {
         wake_compute_retry_config: config::RetryConfig::parse(&args.wake_compute_retry)?,
         connect_compute_locks,
         connect_to_compute: compute_config,
+        greetings,
         #[cfg(feature = "testing")]
         disable_pg_session_jwt: false,
         #[cfg(feature = "rest_broker")]