Add NumberOfNavigateToPullRequestFileDiff metrics

This counter is incremented when GoToSolutionOrPullRequestFileCommand
is used to navigate from a solution file to a PR file diff.

Author: jcansdale

src/GitHub.Exports/Models/UsageModel.cs

@@ -70,6 +70,7 @@ public class MeasuresModel
             public int NumberOfShowCurrentPullRequest { get; set; }
             public int NumberOfStatusBarOpenPullRequestList { get; set; }
             public int NumberOfTeamExplorerHomeOpenPullRequestList { get; set; }
+            public int NumberOfNavigateToPullRequestFileDiff { get; set; }
         }
     }
 }

src/GitHub.VisualStudio/Commands/GoToSolutionOrPullRequestFileCommand.cs

@@ -104,10 +104,8 @@ public override async Task Execute()
                     return;
                 }
 
-                // Navigate from working file to PR diff
-                // TODO: add metrics
-                // await usageTracker.Value.IncrementCounter(x => x.NumberOf???);
-
+                // Navigate from solution file to PR diff file
+                await usageTracker.Value.IncrementCounter(x => x.NumberOfNavigateToPullRequestFileDiff);
 
                 var diffViewer = await pullRequestEditorService.Value.OpenDiff(session, relativePath, "HEAD", scrollToFirstDiff: false);
                 if (diffViewer == null)