Add kubernetes_agents usage metric

This currently is a total count of agents. More detailed metrics to come

changelogs/unreleased/usage_ping_kubernetes_agent.yml

+---
+title: Add kubernetes_agents usage metric
+merge_request: 40559
+author:
+type: other

lib/gitlab/usage_data.rb

@@ -110,6 +110,7 @@ module Gitlab
             clusters_applications_jupyter: count(::Clusters::Applications::Jupyter.available),
             clusters_applications_cilium: count(::Clusters::Applications::Cilium.available),
             clusters_management_project: count(::Clusters::Cluster.with_management_project),
+            kubernetes_agents: count(::Clusters::Agent),
             in_review_folder: count(::Environment.in_review_folder),
             grafana_integrated_projects: count(GrafanaIntegration.enabled),
             groups: count(Group),

spec/factories/usage_data.rb

@@ -65,6 +65,9 @@ FactoryBot.define do
       create(:alert_management_alert, issue: alert_bot_issues[0], project: projects[0])
       create(:self_managed_prometheus_alert_event, related_issues: [issues[1]], project: projects[0])
 
+      # Kubernetes agents
+      create(:cluster_agent, project: projects[0])
+
       # Enabled clusters
       gcp_cluster = create(:cluster_provider_gcp, :created).cluster
       create(:cluster_provider_aws, :created)

spec/lib/gitlab/usage_data_spec.rb

@@ -408,6 +408,7 @@ RSpec.describe Gitlab::UsageData, :aggregate_failures do
       expect(count_data[:clusters_applications_jupyter]).to eq(1)
       expect(count_data[:clusters_applications_cilium]).to eq(1)
       expect(count_data[:clusters_management_project]).to eq(1)
+      expect(count_data[:kubernetes_agents]).to eq(1)
 
       expect(count_data[:deployments]).to eq(4)
       expect(count_data[:successful_deployments]).to eq(2)