Add PR reviews metrics.

[grokys]

Adds two new metrics:

• NumberOfPRReviewDiffViewInlineCommentStartReview: incremented when the "Start Review" button is clicked from an inline comment
• NumberOfPRReviewPosts: incremented when a PR review is submitted

Depends on #1562
Part of #1491

[meaghanlewis]

These metrics look good to me! Tested on VS2015 & VS2017

[drguthals]

The first time I started a review from a diff and submitted that review it did not get recorded. I've tried twice since then and it worked. I'm going to try re-starting my experimental instance to see if the first one works when I re-start.

[drguthals]

Tests:
2015 Dogfood:

• responding to a comment to start a review did not cause the comment to remain open
• all metrics were updated

2015 Experimental:

• responding to a comment to start a review did not cause the comment to remain open
• all metrics were updated
• This time I submitted as comment only and it increased numberOfPRReviewPosts - which I believe is intended behavior, I just wanted to make sure 👍

2017 Experimental:

• responding to a comment to start a review did not cause the comment to remain open
• all metrics were updated
• This time I submitted as approved and it still worked 👍

[drguthals]

Looks good! Thanks for fixing that one bug. I have not been able to repro the first time that my metrics weren't updated, but I'm going to assume that was just funky with me for now.