Show Ajax requests in performance bar

But first, rewrite the performance bar in Vue:

1. Remove the peek-host gem and replace it with existing code. This also allows
   us to include the host in the JSON response, rather than in the page HTML.
2. Leave the line profiler parts as here-be-dragons: nicer would be a separate
   endpoint for these, so we could use them on Ajax requests too.
3. The performance bar is too fiddly to rewrite right now, so apply the same
   logic to that.

Then, add features! All requests made through Axios are able to be tracked. To
keep a lid on memory usage, only the first two requests for a given URL are
tracked, though. Each request that's tracked has the same data as the initial
page load, with the exception of the performance bar and the line profiler, as
explained above.

1 files changed, 5 insertions, 1 deletions

diff --git a/doc/administration/monitoring/performance/performance_bar.md b/doc/administration/monitoring/performance/performance_bar.md
index ec1cbce1bad..dc4f685d843 100644
--- a/doc/administration/monitoring/performance/performance_bar.md
+++ b/doc/administration/monitoring/performance/performance_bar.md
@@ -13,12 +13,16 @@ It allows you to see (from left to right):
 ![SQL profiling using the Performance Bar](img/performance_bar_sql_queries.png)
 - time taken and number of [Gitaly] calls, click through for details of these calls
 ![Gitaly profiling using the Performance Bar](img/performance_bar_gitaly_calls.png)
-- profile of the code used to generate the page, line by line for either _all_, _app & lib_ , or _views_. In the profile view, the numbers in the left panel represent wall time, cpu time, and number of calls (based on [rblineprof](https://github.com/tmm1/rblineprof)).
+- profile of the code used to generate the page, line by line. In the profile view, the numbers in the left panel represent wall time, cpu time, and number of calls (based on [rblineprof](https://github.com/tmm1/rblineprof)).
 ![Line profiling using the Performance Bar](img/performance_bar_line_profiling.png)
 - time taken and number of calls to Redis
 - time taken and number of background jobs created by Sidekiq
 - time taken and number of Ruby GC calls
 
+On the far right is a request selector that allows you to view the same metrics
+(excluding the page timing and line profiler) for any requests made while the
+page was open. Only the first two requests per unique URL are captured.
+
 ## Enable the Performance Bar via the Admin panel
 
 GitLab Performance Bar is disabled by default. To enable it for a given group,