Bug 1436680 - Allow non-templated uses of AutoCounter. r=Dexter

In order to stay consistent with the AutoTimer api, allowed non-templates uses or AutoCounter.

MozReview-Commit-ID: 9qnAeQTIY9T

--HG--
extra : rebase_source : 8c95992e2d291d628721febbc7b641b91c850288

toolkit/components/telemetry/Telemetry.h

@@ -305,6 +305,41 @@ private:
   MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER
 };
 
+class MOZ_RAII RuntimeAutoCounter
+{
+public:
+  explicit RuntimeAutoCounter(
+    HistogramID aId,
+    uint32_t counterStart = 0 MOZ_GUARD_OBJECT_NOTIFIER_PARAM)
+    : id(aId)
+    , counter(counterStart)
+  {
+    MOZ_GUARD_OBJECT_NOTIFIER_INIT;
+  }
+
+  ~RuntimeAutoCounter()
+  {
+    Accumulate(id, counter);
+  }
+
+  // Prefix increment only, to encourage good habits.
+  void operator++()
+  {
+    ++counter;
+  }
+
+  // Chaining doesn't make any sense, don't return anything.
+  void operator+=(int increment)
+  {
+    counter += increment;
+  }
+
+private:
+  HistogramID id;
+  uint32_t counter;
+  MOZ_DECL_USE_GUARD_OBJECT_NOTIFIER
+};
+
 template<HistogramID id>
 class MOZ_RAII AutoCounter {
 public:

toolkit/components/telemetry/docs/collection/histograms.rst

@@ -331,7 +331,7 @@ The ``Telemetry.h`` header also declares the helper classes ``AutoTimer`` and ``
     return NS_OK;
   }
 
-If the HistogramID is not known at compile time, one can use the ``RuntimeAutoTimer`` class, which behaves like the template parameterized ``AutoTimer``.
+If the HistogramID is not known at compile time, one can use the ``RuntimeAutoTimer`` and ``RuntimeAutoCounter`` classes, which behave like the template parameterized ``AutoTimer`` and ``AutoCounter`` ones.
 
 .. code-block:: cpp
 
@@ -344,4 +344,14 @@ If the HistogramID is not known at compile time, one can use the ``RuntimeAutoTi
     ...
   }
 
+  int32_t
+  FunctionWithCounter(Telemetry::HistogramID aTelemetryID)
+  {
+    ...
+    // Increment an AutoCounter
+    Telemetry::RuntimeAutoCounter myCounter(aTelemetryID);
+    ++myCounter;
+    ...
+  }
+
 Prefer using the template parameterized ``AutoTimer`` and ``AutoCounter`` on hot paths, if possible.