cloud_storage: minimal cache observability fixes

- Update size+count stats continuously, not just
  at the beginning of trim.
- Log info-level messages at start+end of trim.

src/v/cloud_storage/cache_service.cc

@@ -108,6 +108,7 @@ uint64_t cache::get_total_cleaned() { return _total_cleaned; }
 ss::future<> cache::consume_cache_space(size_t sz) {
  vassert(ss::this_shard_id() == 0, "This method can only run on shard 0");
  _current_cache_size += sz;
+ probe.set_size(_current_cache_size);
  if (_current_cache_size > _max_cache_size) {
  if (ss::lowres_clock::now() - _last_clean_up > min_clean_up_interval) {
  auto units = ss::try_get_units(_cleanup_sm, 1);
@@ -178,11 +179,16 @@ ss::future<> cache::clean_up_at_start() {
 ss::future<> cache::clean_up_cache() {
  vassert(ss::this_shard_id() == 0, "Method can only be invoked on shard 0");
  gate_guard guard{_gate};
+
+ vlog(
+ cst_log.info,
+ "Scanning cache prior to trim...");
  auto [current_cache_size, candidates_for_deletion, _]
  = co_await _walker.walk(_cache_dir.native(), _access_time_tracker);
  _current_cache_size = current_cache_size;
  probe.set_size(_current_cache_size);
- probe.set_num_files(candidates_for_deletion.size());
+ auto initial_object_count = candidates_for_deletion.size();
+ probe.set_num_files(initial_object_count);
 
  // Updating the access time tracker in case if some files were removed
  // from cache directory by the user manually.
@@ -195,6 +201,12 @@ ss::future<> cache::clean_up_cache() {
 
  uint64_t deleted_size = 0;
  if (_current_cache_size >= _max_cache_size) {
+ vlog(
+ cst_log.info,
+ "Trimming cache ({} >= {})",
+ _current_cache_size,
+ _max_cache_size);
+
  auto size_to_delete
  = _current_cache_size
  - (_max_cache_size * (long double)_cache_size_low_watermark);
@@ -270,9 +282,11 @@ ss::future<> cache::clean_up_cache() {
  }
  _total_cleaned += deleted_size;
  _current_cache_size -= deleted_size;
+ probe.set_size(_current_cache_size);
+ probe.set_num_files(initial_object_count - i_to_delete);
  vlog(
- cst_log.debug,
- "Cache eviction deleted {} files of total size {}.",
+ cst_log.info,
+ "Trimmed cache: deleted {} files of total size {}.",
  i_to_delete,
  deleted_size);
  }