offload_lfc_interval_seconds in ComputeSpec (#12447)

- Add ComputeSpec flag `offload_lfc_interval_seconds` controlling
  whether LFC should be offloaded to endpoint storage. Default value
  (None) means "don't offload".
- Add glue code around it for `neon_local` and integration tests.
- Add `autoprewarm` mode for `test_lfc_prewarm` testing
  `offload_lfc_interval_seconds` and `autoprewarm` flags in conjunction.
- Rename `compute_ctl_lfc_prewarm_requests_total` and
`compute_ctl_lfc_offload_requests_total` to
`compute_ctl_lfc_prewarms_total`
  and `compute_ctl_lfc_offloads_total` to reflect we count prewarms and
  offloads, not `compute_ctl` requests of those.
  Don't count request in metrics if there is a prewarm/offload already
  ongoing.

https://github.com/neondatabase/cloud/issues/19011
Resolves: https://github.com/neondatabase/cloud/issues/30770

libs/compute_api/src/responses.rs

@@ -58,7 +58,7 @@ pub enum LfcPrewarmState {
     },
 }
 
-#[derive(Serialize, Default, Debug, Clone)]
+#[derive(Serialize, Default, Debug, Clone, PartialEq)]
 #[serde(tag = "status", rename_all = "snake_case")]
 pub enum LfcOffloadState {
     #[default]

libs/compute_api/src/spec.rs

@@ -181,10 +181,14 @@ pub struct ComputeSpec {
     /// JWT for authorizing requests to endpoint storage service
     pub endpoint_storage_token: Option<String>,
 
-    /// Download LFC state from endpoint_storage and pass it to Postgres on startup
     #[serde(default)]
+    /// Download LFC state from endpoint storage and pass it to Postgres on compute startup
     pub autoprewarm: bool,
 
+    #[serde(default)]
+    /// Upload LFC state to endpoint storage periodically. Default value (None) means "don't upload"
+    pub offload_lfc_interval_seconds: Option<std::num::NonZeroU64>,
+
     /// Suspend timeout in seconds.
     ///
     /// We use this value to derive other values, such as the installed extensions metric.