Add ObjC API for getting native histograms.

webrtc/sdk/objc/Framework/Headers/WebRTC/RTCMetricsSampleInfo.h

+/*
+ *  Copyright 2016 The WebRTC 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 in the root of the source
+ *  tree. An additional intellectual property rights grant can be found
+ *  in the file PATENTS.  All contributing project authors may
+ *  be found in the AUTHORS file in the root of the source tree.
+ */
+
+#import <Foundation/Foundation.h>
+
+#import <WebRTC/RTCMacros.h>
+
+NS_ASSUME_NONNULL_BEGIN
+
+RTC_EXPORT
+@interface RTCMetricsSampleInfo : NSObject
+
+/**
+ * Example of RTCMetricsSampleInfo:
+ * name: "WebRTC.Video.InputFramesPerSecond"
+ * min: 1
+ * max: 100
+ * bucketCount: 50
+ * samples: [29]:2 [30]:1
+ */
+
+/** The name of the histogram. */
+@property(nonatomic, readonly) NSString* name;
+
+/** The minimum bucket value. */
+@property(nonatomic, readonly) int min;
+
+/** The maximum bucket value. */
+@property(nonatomic, readonly) int max;
+
+/** The number of buckets. */
+@property(nonatomic, readonly) int bucketCount;
+
+/** A dictionary holding the samples <value, # of events>. */
+@property(nonatomic, readonly) NSDictionary<NSNumber*, NSNumber*>* samples;
+
+- (instancetype)init NS_UNAVAILABLE;
+
+@end
+
+NS_ASSUME_NONNULL_END