Conditionally initialize the global opentracing tracer

This change will instantiate an OpenTracing tracer and configure it
as the global tracer when the GITLAB_TRACING environment variable is
configured. GITLAB_TRACING takes a "connection string"-like value,
encapsulating the driver (eg jaeger, etc) and options for the driver.

Since each service, whether it's written in Ruby or Golang, uses the
same connection-string, it should be very easy to configure all
services in a cluster, or even a single development machine to be
setup to use tracing.

Note that this change does not include instrumentation or propagation
changes as this is a way of breaking a previous larger change into
components. The instrumentation and propagation changes will follow
in separate changes.

1 files changed, 17 insertions, 0 deletions

diff --git a/lib/gitlab/tracing.rb b/lib/gitlab/tracing.rb
new file mode 100644
index 00000000000..3c4db42ac06
--- /dev/null
+++ b/lib/gitlab/tracing.rb
@@ -0,0 +1,17 @@
+# frozen_string_literal: true
+
+module Gitlab
+  module Tracing
+    # Only enable tracing when the `GITLAB_TRACING` env var is configured. Note that we avoid using ApplicationSettings since
+    # the same environment variable needs to be configured for Workhorse, Gitaly and any other components which
+    # emit tracing. Since other components may start before Rails, and may not have access to ApplicationSettings,
+    # an env var makes more sense.
+    def self.enabled?
+      connection_string.present?
+    end
+
+    def self.connection_string
+      ENV['GITLAB_TRACING']
+    end
+  end
+end