[debugger] track debugger feature usage via histogram.

src/counters.h

 // Copyright 2012 the V8 project authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #ifndef V8_COUNTERS_H_
 #define V8_COUNTERS_H_
 
 #include "include/v8.h"
 #include "src/allocation.h"
 #include "src/base/platform/elapsed-timer.h"
@@ skipping 466 matching lines @@
 
 
 #define HISTOGRAM_RANGE_LIST(HR)                                              \
   /* Generic range histograms */                                              \
   HR(detached_context_age_in_gc, V8.DetachedContextAgeInGC, 0, 20, 21)        \
   HR(gc_idle_time_allotted_in_ms, V8.GCIdleTimeAllottedInMS, 0, 10000, 101)   \
   HR(gc_idle_time_limit_overshot, V8.GCIdleTimeLimit.Overshot, 0, 10000, 101) \
   HR(gc_idle_time_limit_undershot, V8.GCIdleTimeLimit.Undershot, 0, 10000,    \
      101)                                                                     \
   HR(code_cache_reject_reason, V8.CodeCacheRejectReason, 1, 6, 6)             \
-  HR(errors_thrown_per_context, V8.ErrorsThrownPerContext, 0, 200, 20)
+  HR(errors_thrown_per_context, V8.ErrorsThrownPerContext, 0, 200, 20)        \
+  HR(debug_feature_usage, V8.DebugFeatureUsage, 1, 7, 7)
 
 #define HISTOGRAM_TIMER_LIST(HT)                                              \
   /* Garbage collection timers. */                                            \
   HT(gc_compactor, V8.GCCompactor, 10000, MILLISECOND)                        \
   HT(gc_finalize, V8.GCFinalizeMC, 10000, MILLISECOND)                        \
   HT(gc_finalize_reduce_memory, V8.GCFinalizeMCReduceMemory, 10000,           \
      MILLISECOND)                                                             \
   HT(gc_scavenger, V8.GCScavenger, 10000, MILLISECOND)                        \
   HT(gc_context, V8.GCContext, 10000,                                         \
      MILLISECOND) /* GC context cleanup time */                               \
@@ skipping 393 matching lines @@
 
   explicit Counters(Isolate* isolate);
 
   DISALLOW_IMPLICIT_CONSTRUCTORS(Counters);
 };
 
 }  // namespace internal
 }  // namespace v8
 
 #endif  // V8_COUNTERS_H_