Merge branch '18451-track-new-redis-connections' into 'master'

Resolve "Track the number of new Redis connections per transaction"

## What does this MR do?

Add a new metric counter, `new_redis_connections`, that contains the number of calls to `Redis::Client#connect` in the current transaction.

## Are there points in the code the reviewer needs to double check?

Not sure. I tested this in kind of a brute-force way:

1. Add a debugger in the monkey-patched `connect` method.
2. With metrics enabled, start the app and load a page.
3. The first Redis connection is created by `Rack::Attack` and isn't in a transaction, but still works fine.
4. The second Redis connection is within a transaction (the page load), and increments the counter.
5. If I reload the page, neither debugger is hit.
6. If I use a Redis client and do `CLIENT KILL` on my two existing clients, then reload the page, I get 3 and 4 again.
7. If I disable metrics collection, the debugger never gets hit.

## Why was this MR needed?

We may have a Redis connection leak somewhere, so adding metrics will let us track this.

## What are the relevant issue numbers?

Closes #18451.

## Screenshots (if relevant)

Hahaha nope, not relevant.

## Does this MR meet the acceptance criteria?

- [ ] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- [ ] Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

cc @yorickpeterse 

See merge request !4649

CHANGELOG

@@ -24,6 +24,7 @@ v 8.9.0 (unreleased)
   - Reduce number of fog gem dependencies
   - Remove project notification settings associated with deleted projects
   - Fix 404 page when viewing TODOs that contain milestones or labels in different projects
+  - Add a metric for the number of new Redis connections created by a transaction
   - Redesign navigation for project pages
   - Fix groups API to list only user's accessible projects
   - Redesign account and email confirmation emails

config/initializers/metrics.rb

@@ -133,4 +133,20 @@ if Gitlab::Metrics.enabled?
   GC::Profiler.enable
 
   Gitlab::Metrics::Sampler.new.start
+
+  module TrackNewRedisConnections
+    def connect(*args)
+      val = super
+
+      if current_transaction = Gitlab::Metrics::Transaction.current
+        current_transaction.increment(:new_redis_connections, 1)
+      end
+
+      val
+    end
+  end
+
+  class ::Redis::Client
+    prepend TrackNewRedisConnections
+  end
 end

spec/requests/git_http_spec.rb

@@ -340,7 +340,7 @@ describe 'Git HTTP requests', lib: true do
       end
     end
 
-    context "when the file exists" do
+    context "when the file does not exist" do
       before { get "/#{project.path_with_namespace}/blob/master/info/refs" }
 
       it "returns not found" do