make session_id a first class member of Dispatcher

previously, we'd generate a trace_id only in the SmtpDispatcher
for the tracing subsystem.

We want to capture a session id in the logs, so the first step
is to formalize this field.

refs: https://github.com/KumoCorp/kumomta/issues/316

crates/kumod/src/ready_queue.rs

@@ -39,6 +39,7 @@ use throttle::ThrottleSpec;
 use tokio::sync::Notify;
 use tokio::task::JoinHandle;
 use tracing::instrument; // TODO move to here
+use uuid::Uuid;
 
 static MANAGER: LazyLock<StdMutex<ReadyQueueManager>> =
     LazyLock::new(|| StdMutex::new(ReadyQueueManager::new()));
@@ -933,6 +934,7 @@ pub struct Dispatcher {
     pub msgs: Vec<Message>,
     pub delivery_protocol: String,
     pub suspended: Option<AdminSuspendReadyQEntryRef>,
+    pub session_id: Uuid,
     leases: Vec<LimitLease>,
     batch_started: Option<tokio::time::Instant>,
 }
@@ -1029,6 +1031,7 @@ impl Dispatcher {
             leases,
             suspended: None,
             batch_started: None,
+            session_id: Uuid::new_v4(),
         };
 
         let mut queue_dispatcher: Box<dyn QueueDispatcher> = match &queue_config.borrow().protocol {

crates/kumod/src/smtp_dispatcher.rs

@@ -25,7 +25,6 @@ use serde::{Deserialize, Serialize};
 use std::net::SocketAddr;
 use std::sync::{Arc, LazyLock};
 use tracing::Level;
-use uuid::Uuid;
 
 static BROKEN_TLS_BY_SITE: LazyLock<LruCacheWithTtl<String, ()>> =
     LazyLock::new(|| LruCacheWithTtl::new_named("smtp_dispatcher_broken_tls", 64 * 1024));
@@ -97,8 +96,6 @@ impl SmtpDispatcher {
             },
         };
 
-        let trace_id = Uuid::new_v4().to_string();
-
         let addresses = if proto_config.mx_list.is_empty() {
             dispatcher
                 .mx
@@ -128,7 +125,7 @@ impl SmtpDispatcher {
         let tracer = Arc::new(SmtpClientTracerImpl::new(serde_json::json!({
             "egress_pool": dispatcher.egress_pool.to_string(),
             "egress_source": dispatcher.egress_source.name.to_string(),
-            "id": trace_id,
+            "id": dispatcher.session_id.to_string(),
             "ready_queue_name": dispatcher.name.to_string(),
             "mx_plan": addresses.clone(),
         })));