Do not use value arg to hold chart type

According to snowplow docs, value has to be an Int or Float.

lib/gitlab/metrics/dashboard/stages/track_panel_type.rb

@@ -18,7 +18,7 @@ module Gitlab
           def track_panel_type(panel)
             panel_type = panel[:type]
 
-            Gitlab::Tracking.event('MetricsDashboard::Chart', 'chart_rendered', label: 'Chart Type', value: panel_type)
+            Gitlab::Tracking.event('MetricsDashboard::Chart', 'chart_rendered', label: panel_type)
           end
         end
       end

spec/lib/gitlab/metrics/dashboard/stages/track_panel_type_spec.rb

@@ -14,10 +14,14 @@ RSpec.describe Gitlab::Metrics::Dashboard::Stages::TrackPanelType do
     let(:dashboard) { load_sample_dashboard.deep_symbolize_keys }
 
     it 'creates tracking event' do
-      expect(Gitlab::Tracking).to receive(:event).with('MetricsDashboard::Chart', 'chart_rendered',
-        { label: 'Chart Type', value: 'area-chart' }).at_least(:once)
+      stub_application_setting(snowplow_enabled: true, snowplow_collector_hostname: 'localhost')
+      allow(Gitlab::Tracking).to receive(:event).and_call_original
 
       subject.transform!
+
+      expect(Gitlab::Tracking).to have_received(:event)
+        .with('MetricsDashboard::Chart', 'chart_rendered', { label: 'area-chart' })
+        .at_least(:once)
     end
   end
 end

spec/services/metrics/dashboard/custom_dashboard_service_spec.rb

@@ -58,11 +58,13 @@ RSpec.describe Metrics::Dashboard::CustomDashboardService, :use_clean_rails_memo
       end
 
       it 'tracks panel type' do
-        expect(::Gitlab::Tracking).to receive(:event).with(
-          'MetricsDashboard::Chart', 'chart_rendered', { label: 'Chart Type', value: 'area-chart' }
-        ).at_least(:once)
+        allow(::Gitlab::Tracking).to receive(:event).and_call_original
 
         described_class.new(*service_params).get_dashboard
+
+        expect(::Gitlab::Tracking).to have_received(:event)
+          .with('MetricsDashboard::Chart', 'chart_rendered', { label: 'area-chart' })
+          .at_least(:once)
       end
 
       context 'and the dashboard is then deleted' do