config: added a property controlling max size of readers cache

Added property which allows controlling maximum number of readers kept
in cache per ntp. The value was previously unbounded which may lead to
a situation in which readers cache grew and caused out of memory error.

Signed-off-by: Michal Maslanka <michal@redpanda.com>

src/v/config/configuration.cc

@@ -86,6 +86,15 @@ configuration::configuration()
       "Duration after which inactive readers will be evicted from cache",
       {.visibility = visibility::tunable},
       30s)
+  , readers_cache_target_max_size(
+      *this,
+      "readers_cache_target_max_size",
+      "Maximum desired number of readers cached per ntp. This a soft limit, a "
+      "number of readers in cache may temporary increase as cleanup is done in "
+      "background",
+      {.needs_restart = needs_restart::no, .visibility = visibility::tunable},
+      200,
+      {.min = 0, .max = 10000})
   , log_segment_ms(
       *this,
       "log_segment_ms",

src/v/config/configuration.h

@@ -51,6 +51,8 @@ struct configuration final : public config_store {
     bounded_property<uint16_t> log_segment_size_jitter_percent;
     bounded_property<uint64_t> compacted_log_segment_size;
     property<std::chrono::milliseconds> readers_cache_eviction_timeout_ms;
+    bounded_property<size_t> readers_cache_target_max_size;
+
     bounded_property<std::optional<std::chrono::milliseconds>> log_segment_ms;
     bounded_property<std::chrono::milliseconds> log_segment_ms_min;
     bounded_property<std::chrono::milliseconds> log_segment_ms_max;