Added the URLSessionTaskMetrics support for downloader && operation, which can be used for network metrics

SDWebImage/Core/SDWebImageDownloader.m

@@ -498,6 +498,15 @@ didReceiveResponse:(NSURLResponse *)response
     }
 }
 
+- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didFinishCollectingMetrics:(NSURLSessionTaskMetrics *)metrics API_AVAILABLE(macosx(10.12), ios(10.0), watchos(3.0), tvos(10.0)) {
+    
+    // Identify the operation that runs this task and pass it the delegate method
+    NSOperation<SDWebImageDownloaderOperation> *dataOperation = [self operationWithTask:task];
+    if ([dataOperation respondsToSelector:@selector(URLSession:task:didFinishCollectingMetrics:)]) {
+        [dataOperation URLSession:session task:task didFinishCollectingMetrics:metrics];
+    }
+}
+
 @end
 
 @implementation SDWebImageDownloadToken

SDWebImage/Core/SDWebImageDownloaderOperation.h

@@ -36,6 +36,7 @@
 
 @optional
 @property (strong, nonatomic, readonly, nullable) NSURLSessionTask *dataTask;
+@property (strong, nonatomic, readonly, nullable) NSURLSessionTaskMetrics *metrics API_AVAILABLE(macosx(10.12), ios(10.0), watchos(3.0), tvos(10.0));
 @property (strong, nonatomic, nullable) NSURLCredential *credential;
 @property (assign, nonatomic) double minimumProgressInterval;
 
@@ -62,6 +63,12 @@
  */
 @property (strong, nonatomic, readonly, nullable) NSURLSessionTask *dataTask;
 
+/**
+ * The collected metrics from `-URLSession:task:didFinishCollectingMetrics:`.
+ * This can be used to collect the network metrics like download duration, DNS lookup duration, SSL handshake dureation, etc. See Apple's documentation: https://developer.apple.com/documentation/foundation/urlsessiontaskmetrics
+ */
+@property (strong, nonatomic, readonly, nullable) NSURLSessionTaskMetrics *metrics API_AVAILABLE(macosx(10.12), ios(10.0), watchos(3.0), tvos(10.0));
+
 /**
  * The credential used for authentication challenges in `-URLSession:task:didReceiveChallenge:completionHandler:`.
  *

SDWebImage/Core/SDWebImageDownloaderOperation.m

@@ -52,6 +52,8 @@ typedef NSMutableDictionary<NSString *, id> SDCallbacksDictionary;
 
 @property (strong, nonatomic, readwrite, nullable) NSURLSessionTask *dataTask;
 
+@property (strong, nonatomic, readwrite, nullable) NSURLSessionTaskMetrics *metrics API_AVAILABLE(macosx(10.12), ios(10.0), watchos(3.0), tvos(10.0));
+
 @property (strong, nonatomic, nonnull) dispatch_queue_t coderQueue; // the queue to do image decoding
 #if SD_UIKIT
 @property (assign, nonatomic) UIBackgroundTaskIdentifier backgroundTaskId;
@@ -512,6 +514,10 @@ didReceiveResponse:(NSURLResponse *)response
     }
 }
 
+- (void)URLSession:(NSURLSession *)session task:(NSURLSessionTask *)task didFinishCollectingMetrics:(NSURLSessionTaskMetrics *)metrics API_AVAILABLE(macosx(10.12), ios(10.0), watchos(3.0), tvos(10.0)) {
+    self.metrics = metrics;
+}
+
 #pragma mark Helper methods
 + (SDWebImageOptions)imageOptionsFromDownloaderOptions:(SDWebImageDownloaderOptions)downloadOptions {
     SDWebImageOptions options = 0;