DEV: Log number of live slots used by requests (#29884)

config/initializers/101-lograge.rb

@@ -109,26 +109,27 @@ if ENV["ENABLE_LOGSTASH_LOGGER"] == "1"
             }
 
             if data = (Thread.current[:_method_profiler] || event.payload[:timings])
-              sql = data[:sql]
-
-              if sql
+              if sql = data[:sql]
                 output[:db] = sql[:duration] * 1000
                 output[:db_calls] = sql[:calls]
               end
 
-              redis = data[:redis]
-
-              if redis
+              if redis = data[:redis]
                 output[:redis] = redis[:duration] * 1000
                 output[:redis_calls] = redis[:calls]
               end
 
-              net = data[:net]
-
-              if net
+              if net = data[:net]
                 output[:net] = net[:duration] * 1000
                 output[:net_calls] = net[:calls]
               end
+
+              # MethodProfiler.stop is called after this lambda, so the delta
+              # must be computed here.
+              if data[:__start_gc_heap_live_slots]
+                output[:heap_live_slots] = GC.stat[:heap_live_slots] -
+                  data[:__start_gc_heap_live_slots]
+              end
             end
 
             output

lib/method_profiler.rb

@@ -90,7 +90,10 @@ class MethodProfiler
 
   def self.start(transfer = nil)
     Thread.current[:_method_profiler] = transfer ||
-      { __start: Process.clock_gettime(Process::CLOCK_MONOTONIC) }
+      {
+        __start: Process.clock_gettime(Process::CLOCK_MONOTONIC),
+        __start_gc_heap_live_slots: GC.stat[:heap_live_slots],
+      }
   end
 
   def self.clear