Include the ReportFlags in logging samples

Summary:
Log the ReportFlags in Haxl Env to some data sets. This can help us group
samples by differnet report flags and check stats like the allocation overhead
for enabling stats or profiling.

Reviewed By: josefs

Differential Revision: D33436458

fbshipit-source-id: 092e36b361e0f27ae061d8d6c81b7fdf467bfb61

Author: watashi

Haxl/Core/Flags.hs

@@ -32,6 +32,7 @@ module Haxl.Core.Flags
 import Control.Monad
 import Data.Bits
 import Data.List (foldl')
+import Text.Printf (printf)
 
 -- ---------------------------------------------------------------------------
 -- ReportFlags
@@ -51,6 +52,11 @@ profilingDependents =
 
 newtype ReportFlags = ReportFlags Int
 
+instance Show ReportFlags where
+  show (ReportFlags fs) = printf "%0*b" (fromEnum maxReportFlag + 1) fs
+    where
+      maxReportFlag = maxBound :: ReportFlag
+
 defaultReportFlags :: ReportFlags
 defaultReportFlags = ReportFlags 0