Fix metric files being wiped after the app starts

When we hit our app with the initial request, in `warmup`,
some metrics already being created as well as corresponding files.
If we do `multiproc_file_dir` cleanup after that, we delete the files
from the dir while keeping them in memory which leads to the incorrect
behavior: the metric is being updated in in-memory, while is not present
in the db, not sent to Prometheus as the result.

1 files changed, 15 insertions, 0 deletions

diff --git a/config.ru b/config.ru
index 6f6fb85d8fa..d3c6e304998 100644
--- a/config.ru
+++ b/config.ru
@@ -17,7 +17,22 @@ end
 
 require ::File.expand_path('../config/environment', __FILE__)
 
+
+# The following is necessary to ensure stale Prometheus metrics don't accumulate over time.
+# It needs to be done as early as here to ensure metrics files aren't deleted.
+# After we hit our app in `warmup`, first metrics and corresponding files already being created,
+# for example in `lib/gitlab/metrics/requests_rack_middleware.rb`.
+def cleanup_prometheus_multiproc_dir
+  if dir = ::Prometheus::Client.configuration.multiprocess_files_dir
+    old_metrics = Dir[File.join(dir, '*.db')]
+
+    FileUtils.rm_rf(old_metrics)
+  end
+end
+
 warmup do |app|
+  cleanup_prometheus_multiproc_dir
+
   client = Rack::MockRequest.new(app)
   client.get('/')
 end