Avoid per-key hash lookup when excluded_keys is empty (#318)

* Avoid per-key hash lookup when excluded_keys is empty

Store nil instead of an empty hash when no excluded_keys are configured
(the default) and use safe navigation (&.key?) in the hot loop. This
avoids an unnecessary hash lookup for every key at every level of the
statistics tree.

Benchmarked at ~39% faster for the default (empty) case with ~200 keys,
with no penalty when exclusions are configured.

* Skip hash construction when excluded_keys is empty

Avoid allocating and freezing an empty hash in the constructor when no
exclusions are configured, as suggested by Copilot review.

Author: mensfeld

lib/karafka/core/monitoring/statistics_decorator.rb

@@ -34,8 +34,11 @@ def initialize(excluded_keys: [])
           @previous_at = monotonic_now.round
           # Cache for memoized suffix keys to avoid repeated string allocations
           @suffix_keys_cache = {}
-          # Frozen hash for O(1) key exclusion lookup
-          @excluded_keys = excluded_keys.each_with_object({}) { |k, h| h[k] = true }.freeze
+          # Frozen hash for O(1) key exclusion lookup, nil when empty to avoid per-key
+          # lookups in the hot loop when no exclusions are configured
+          @excluded_keys = unless excluded_keys.empty?
+            excluded_keys.each_with_object({}) { |k, h| h[k] = true }.freeze
+          end
         end
 
         # @param emited_stats [Hash] original emited statistics
@@ -87,7 +90,7 @@ def diff(previous, current, pw, pw_start, change_d)
           pw_size = pw_start
 
           current.each_pair do |key, value|
-            next if excluded.key?(key)
+            next if excluded&.key?(key)
 
             if value.is_a?(Hash)
               diff(filled_previous[key], value, pw, pw_size, change_d)