Make UMA stats creation available in the Java interface.

Only has counts stats right now but enumeration stats can easily be added in the future if needed.

BUG=webrtc:6313

Review-Url: https://codereview.webrtc.org/2320473002
Cr-Commit-Position: refs/heads/master@{#14146}

webrtc/api/android/java/src/org/webrtc/Metrics.java

@@ -30,6 +30,8 @@ import java.util.Map;
 // The metrics can for example be retrieved when a peer connection is closed.
 
 public class Metrics {
+  private static final String TAG = "Metrics";
+
   static {
     System.loadLibrary("jingle_peerconnection_so");
   }
@@ -57,6 +59,36 @@ public class Metrics {
     }
   }
 
+  /**
+   * Class for holding the native pointer of a histogram. Since there is no way to destroy a
+   * histogram, please don't create unnecessary instances of this object. This class is thread safe.
+   *
+   * Usage example:
+   * private static final Histogram someMetricHistogram =
+   *     Histogram.createCounts("WebRTC.Video.SomeMetric", 1, 10000, 50);
+   * someMetricHistogram.addSample(someVariable);
+   */
+  static class Histogram {
+    private final long handle;
+    private final String name; // Only used for logging.
+
+    private Histogram(long handle, String name) {
+      this.handle = handle;
+      this.name = name;
+    }
+
+    static public Histogram createCounts(String name, int min, int max, int bucketCount) {
+      return new Histogram(nativeCreateCounts(name, min, max, bucketCount), name);
+    }
+
+    public void addSample(int sample) {
+      nativeAddSample(handle, sample);
+    }
+
+    private static native long nativeCreateCounts(String name, int min, int max, int bucketCount);
+    private static native void nativeAddSample(long handle, int sample);
+  }
+
   private void add(String name, HistogramInfo info) {
     map.put(name, info);
   }

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 jlongFromPointer(
+      webrtc::metrics::HistogramFactoryGetCounts(name, min, max, buckets));
+}
+
+JOW(void, Metrics_00024Histogram_nativeAddSample)
+(JNIEnv* jni, jclass, jlong histogram, jint sample) {
+  HistogramAdd(reinterpret_cast<webrtc::metrics::Histogram*>(histogram),
+               sample);
+}
+
 }  // namespace webrtc_jni

webrtc/system_wrappers/include/metrics_default.h

@@ -18,6 +18,10 @@
 namespace webrtc {
 namespace metrics {
 
+// This class does not actually exist. It is casted to an implementation defined
+// pointer inside the functions.
+class Histogram;
+
 struct SampleInfo {
   SampleInfo(const std::string& name, int min, int max, size_t bucket_count);
   ~SampleInfo();
@@ -51,6 +55,9 @@ int NumSamples(const std::string& name);
 // Returns the minimum sample value (or -1 if the histogram has no samples).
 int MinSample(const std::string& name);
 
+// Function for adding a |sample| to a histogram without checkking the name.
+void HistogramAdd(Histogram* histogram_pointer, int sample);
+
 }  // namespace metrics
 }  // namespace webrtc
 

webrtc/system_wrappers/source/metrics_default.cc

@@ -244,6 +244,15 @@ void HistogramAdd(Histogram* histogram_pointer,
   ptr->Add(sample);
 }
 
+// Fast path. Adds |sample| to cached |histogram_pointer|.
+void HistogramAdd(Histogram* histogram_pointer, int sample) {
+  if (!histogram_pointer)
+    return;
+
+  RtcHistogram* ptr = reinterpret_cast<RtcHistogram*>(histogram_pointer);
+  ptr->Add(sample);
+}
+
 SampleInfo::SampleInfo(const std::string& name,
                        int min,
                        int max,