Make UMA stats creation available in the Java interface.

webrtc/api/android/jni/androidmetrics_jni.cc

Index: webrtc/api/android/jni/androidmetrics_jni.cc
diff --git a/webrtc/api/android/jni/androidmetrics_jni.cc b/webrtc/api/android/jni/androidmetrics_jni.cc
index 6e658c886bf24d257aa4f5ef9704a537b9e6210f..947227bfce4ab1e717b8b1865876be1d1c5c3364 100644
--- a/webrtc/api/android/jni/androidmetrics_jni.cc
+++ b/webrtc/api/android/jni/androidmetrics_jni.cc
@@ -14,10 +14,12 @@
 #include "webrtc/api/android/jni/classreferenceholder.h"
 #include "webrtc/api/android/jni/jni_helpers.h"
 #include "webrtc/api/android/jni/native_handle_impl.h"
+#include "webrtc/system_wrappers/include/metrics.h"
 #include "webrtc/system_wrappers/include/metrics_default.h"
 
-// Enables collection of native histograms.
+// Enables collection of native histograms and creating them.
 namespace webrtc_jni {
+
 JOW(void, Metrics_nativeEnable)(JNIEnv* jni, jclass) {
   webrtc::metrics::Enable();
 }
@@ -56,4 +58,18 @@ JOW(jobject, Metrics_nativeGetAndReset)(JNIEnv* jni, jclass) {
   CHECK_EXCEPTION(jni);
   return j_metrics;
 }
+
+JOW(jlong, Metrics_00024Histogram_nativeCreateCounts)
+(JNIEnv* jni, jclass, jstring j_name, jint min, jint max, jint buckets) {
+  std::string name = JavaToStdString(jni, j_name);
+  return (jlong)webrtc::metrics::HistogramFactoryGetCounts(name, min, max,

[magjed_webrtc, 2016/09/08 10:19:54]

Use jlongFromPointer when converting a pointer to a jlong.

[sakal, 2016/09/08 11:20:28]

Done.

+                                                           buckets);
+}
+
+JOW(void, Metrics_00024Histogram_nativeAddSample)
+(JNIEnv* jni, jclass, jlong histogram, jint sample) {
+  HistogramAdd(reinterpret_cast<webrtc::metrics::Histogram*>(histogram),

[magjed_webrtc, 2016/09/08 10:19:54]

This is a new function to the histogram interface. According to the
documentation, WebRTC clients must either:
 - provide implementations of the Histogram
 - or link with the default implementations (i.e.
   system_wrappers/system_wrappers.gyp:metrics_default).

So this feels scary if some client link to their own implementation. I would
sleep a little bit better if we use the existing HistogramAdd function, and
store the name string in Metrics.Histogram. We can push for officially adding a
new HistogramAdd function, and update this code when that function becomes
available.

[sakal, 2016/09/08 11:20:29]

The reason I added this function is that I didn't want to pass the name from the
Java side on every addSample call. That's something we wanted to avoid because
JavaToStdString is probably expensive.

This file is already broken if client decides to use any other implementation
that metrics_default. It relies on for example webrtc::metrics::GetAndReset that
is declared in metrics_default.h. I can declare this new method in
metrics_default.h as well if that is better in your opinion.

Code not using this class wouldn't be broken by adding the method to metrics.h
because it is valid to declare a method and never define it as long as you don't
use it.

[magjed_webrtc, 2016/09/08 11:53:29]

I see. I didn't realize we had already committed to using metrics_default for
the Java interface.

Yeah, move the new function declaration to metrics_default.h and leave metrics.h
untouched.

+               sample);
+}
+
 }  // namespace webrtc_jni