cloud_storage: Implement "carryover" cache trimming mechanism

src/v/cloud_storage/cache_probe.cc

@@ -102,6 +102,11 @@ cache_probe::cache_probe() {
                     "Number of times we couldn't free enough space with a fast "
                     "trim and had to fall back to a slower exhaustive trim."))
                   .aggregate(aggregate_labels),
+                sm::make_counter(
+                  "carryover_trims",
+                  [this] { return _carryover_trims; },
+                  sm::description("Number of times we invoked carryover trim."))
+                  .aggregate(aggregate_labels),
                 sm::make_counter(
                   "failed_trims",
                   [this] { return _failed_trims; },

src/v/cloud_storage/cache_probe.h

@@ -39,6 +39,7 @@ class cache_probe {
 
     void fast_trim() { ++_fast_trims; }
     void exhaustive_trim() { ++_exhaustive_trims; }
+    void carryover_trim() { ++_carryover_trims; }
     void failed_trim() { ++_failed_trims; }
 
 private:
@@ -55,6 +56,7 @@ class cache_probe {
 
     int64_t _fast_trims{0};
     int64_t _exhaustive_trims{0};
+    int64_t _carryover_trims{0};
     int64_t _failed_trims{0};
 
     metrics::internal_metric_groups _metrics;