Fix test_lfc_prewarm: reduce number of prewarms, sleep before LFC offloading (#12515)

Fixes:
- Sleep before LFC offloading in `test_lfc_prewarm[autoprewarm]` to
ensure offloaded LFC is the one exported after all writes finish
- Reduce number of prewarms and increase timeout in
`test_lfc_prewarm_under_workload` as debug builds were failing due to
timeout.

Additional changes:
- Remove `check_pinned_entries`:
https://github.com/neondatabase/neon/pull/12447#discussion_r2185946210
- Fix LFC error metrics description:
https://github.com/neondatabase/neon/pull/12486#discussion_r2190763107

compute_tools/src/metrics.rs

@@ -108,7 +108,7 @@ pub(crate) static LFC_PREWARMS: Lazy<IntCounter> = Lazy::new(|| {
 pub(crate) static LFC_PREWARM_ERRORS: Lazy<IntCounter> = Lazy::new(|| {
     register_int_counter!(
         "compute_ctl_lfc_prewarm_errors_total",
-        "Total number of LFC prewarms errors requested by compute_ctl or autoprewarm option",
+        "Total number of LFC prewarm errors",
     )
     .expect("failed to define a metric")
 });
@@ -124,7 +124,7 @@ pub(crate) static LFC_OFFLOADS: Lazy<IntCounter> = Lazy::new(|| {
 pub(crate) static LFC_OFFLOAD_ERRORS: Lazy<IntCounter> = Lazy::new(|| {
     register_int_counter!(
         "compute_ctl_lfc_offload_errors_total",
-        "Total number of LFC offload errors requested by compute_ctl or lfc_offload_period_seconds option",
+        "Total number of LFC offload errors",
     )
     .expect("failed to define a metric")
 });