Fix Snowplow spec helper

With Ruby 2.6 using `**kwargs` here will not work because it would be an
empty hash that we then expect to be submitted to
`Gitlab::Tracking#event`.

An example when using `expect_snowplow_event`:

```
expect_snowplow_event(
  category: 'cluster:services:prometheus',
  action: 'enabled_manual_prometheus'
)
```

Error:
```
Gitlab::Tracking received :event with unexpected arguments
expected:
  ("cluster:services:prometheus", "enabled_manual_prometheus", {})
got:
  ("cluster:services:prometheus", "disabled_manual_prometheus") (1 time)
("cluster:services:prometheus", "enabled_manual_prometheus") (1 time)
```

This has not been an issue so far because the other specs we migrated
always provided additional `kwargs`.

To prevent this, we only expect the `**kwargs` parameter when it exists.
However, this will get fixed with Ruby 2.7 and we can remove the
condition again.

spec/models/project_services/prometheus_service_spec.rb

@@ -2,7 +2,7 @@
 
 require 'spec_helper'
 
-RSpec.describe PrometheusService, :use_clean_rails_memory_store_caching do
+RSpec.describe PrometheusService, :use_clean_rails_memory_store_caching, :snowplow do
   include PrometheusHelpers
   include ReactiveCachingHelpers
 
@@ -421,18 +421,16 @@ RSpec.describe PrometheusService, :use_clean_rails_memory_store_caching do
     context "enabling manual_configuration" do
       it "tracks enable event" do
         service.update!(manual_configuration: false)
-
-        expect(Gitlab::Tracking).to receive(:event).with('cluster:services:prometheus', 'enabled_manual_prometheus')
-
         service.update!(manual_configuration: true)
+
+        expect_snowplow_event(category: 'cluster:services:prometheus', action: 'enabled_manual_prometheus')
       end
 
       it "tracks disable event" do
         service.update!(manual_configuration: true)
-
-        expect(Gitlab::Tracking).to receive(:event).with('cluster:services:prometheus', 'disabled_manual_prometheus')
-
         service.update!(manual_configuration: false)
+
+        expect_snowplow_event(category: 'cluster:services:prometheus', action: 'disabled_manual_prometheus')
       end
     end
   end

spec/support/helpers/snowplow_helpers.rb

@@ -32,8 +32,16 @@ module SnowplowHelpers
   #     end
   #   end
   def expect_snowplow_event(category:, action:, **kwargs)
-    expect(Gitlab::Tracking).to have_received(:event)
-      .with(category, action, **kwargs).at_least(:once)
+    # This check will no longer be needed with Ruby 2.7 which
+    # would not pass any arguments when using **kwargs.
+    # https://gitlab.com/gitlab-org/gitlab/-/issues/263430
+    if kwargs.present?
+      expect(Gitlab::Tracking).to have_received(:event)
+        .with(category, action, **kwargs).at_least(:once)
+    else
+      expect(Gitlab::Tracking).to have_received(:event)
+        .with(category, action).at_least(:once)
+    end
   end
 
   # Asserts that no call to `Gitlab::Tracking#event` was made.