engine.main: fix memory leak due to latency histogram creation

src/core/app.lua

@@ -50,6 +50,14 @@ function timeline ()
  return timeline_log
 end
 
+-- Breath latency histogram
+local latency -- initialized on demand
+function enable_latency_histogram ()
+ if latency == nil then
+ latency = histogram.create('engine/latency.histogram', 1e-6, 1e0)
+ end
+end
+
 -- The set of all active apps and links in the system, indexed by name.
 app_table, link_table = {}, {}
 -- Timeline events specific to app instances
@@ -529,10 +537,10 @@ function main (options)
  -- Ensure timeline is created and initialized
  timeline()
 
- -- Enable latency histogram if requested
+ -- Enable latency histogram unless explicitly disabled
  local breathe = breathe
  if options.measure_latency or options.measure_latency == nil then
- local latency = histogram.create('engine/latency.histogram', 1e-6, 1e0)
+ enable_latency_histogram()
  breathe = latency:wrap_thunk(breathe, now)
  end