[compute_ctl] Do not initialize last_active on start (#4137)

Our scale-to-zero logic was optimized for short auto-suspend intervals,
e.g. minutes or hours. In this case, if compute was restarted by k8s due
to some reason (OOM, k8s node went down, pod relocation, etc.),
`last_active` got bumped, we start counting auto-suspend timeout again.
It's not a big deal, i.e. we suspend completely idle compute not after 5
minutes, but after 10 minutes or so.

Yet, some clients may want days or even weeks. And chance that compute
could be restarted during this interval is pretty high, but in this case
we could be not able to suspend some computes for weeks.

After this commit, we won't initialize `last_active` on start, so
`/status` could return an unset attribute. This means that there was no
user activity since start. Control-plane should deal with it by taking
`max()` out of all available activity timestamps: `started_at`,
`last_active`, etc.

compute_ctl part of neondatabase/cloud#4853

libs/compute_api/src/responses.rs

@@ -19,7 +19,7 @@ pub struct ComputeStatusResponse {
     pub timeline: Option<String>,
     pub status: ComputeStatus,
     #[serde(serialize_with = "rfc3339_serialize")]
-    pub last_active: DateTime<Utc>,
+    pub last_active: Option<DateTime<Utc>>,
     pub error: Option<String>,
 }
 
@@ -29,7 +29,7 @@ pub struct ComputeState {
     pub status: ComputeStatus,
     /// Timestamp of the last Postgres activity
     #[serde(serialize_with = "rfc3339_serialize")]
-    pub last_active: DateTime<Utc>,
+    pub last_active: Option<DateTime<Utc>>,
     pub error: Option<String>,
 }
 
@@ -54,11 +54,15 @@ pub enum ComputeStatus {
     Failed,
 }
 
-fn rfc3339_serialize<S>(x: &DateTime<Utc>, s: S) -> Result<S::Ok, S::Error>
+fn rfc3339_serialize<S>(x: &Option<DateTime<Utc>>, s: S) -> Result<S::Ok, S::Error>
 where
     S: Serializer,
 {
-    x.to_rfc3339().serialize(s)
+    if let Some(x) = x {
+        x.to_rfc3339().serialize(s)
+    } else {
+        s.serialize_none()
+    }
 }
 
 /// Response of the /metrics.json API