Extend billable members count to accept requested hosted plan parameter

- Extended billableMembersCount field to accept requestedHostedPlan
- Added tests for billableMembersCount
- Updates GraphQL docs

Changelog: changed
EE: true

doc/api/graphql/reference/index.md

@@ -11097,7 +11097,6 @@ four standard [pagination arguments](#connection-pagination-arguments):
 | <a id="groupadditionalpurchasedstoragesize"></a>`additionalPurchasedStorageSize` | [`Float`](#float) | Additional storage purchased for the root namespace in bytes. |
 | <a id="groupautodevopsenabled"></a>`autoDevopsEnabled` | [`Boolean`](#boolean) | Indicates whether Auto DevOps is enabled for all projects within this group. |
 | <a id="groupavatarurl"></a>`avatarUrl` | [`String`](#string) | Avatar URL of the group. |
-| <a id="groupbillablememberscount"></a>`billableMembersCount` | [`Int`](#int) | Number of billable users in the group. |
 | <a id="groupcontacts"></a>`contacts` | [`CustomerRelationsContactConnection`](#customerrelationscontactconnection) | Find contacts of this group. (see [Connections](#connections)) |
 | <a id="groupcontainerrepositoriescount"></a>`containerRepositoriesCount` | [`Int!`](#int) | Number of container repositories in the group. |
 | <a id="groupcontainslockedprojects"></a>`containsLockedProjects` | [`Boolean!`](#boolean) | Includes at least one project where the repository size exceeds the limit. |
@@ -11151,6 +11150,18 @@ four standard [pagination arguments](#connection-pagination-arguments):
 
 #### Fields with arguments
 
+##### `Group.billableMembersCount`
+
+Number of billable users in the group.
+
+Returns [`Int`](#int).
+
+###### Arguments
+
+| Name | Type | Description |
+| ---- | ---- | ----------- |
+| <a id="groupbillablememberscountrequestedhostedplan"></a>`requestedHostedPlan` | [`String`](#string) | Plan from which to get billable members. |
+
 ##### `Group.board`
 
 A single board of the group.

ee/app/graphql/ee/types/group_type.rb

@@ -85,7 +85,9 @@ module EE
 
         field :billable_members_count, ::GraphQL::Types::Int,
               null: true,
-              description: 'Number of billable users in the group.'
+              description: 'Number of billable users in the group.' do
+                argument :requested_hosted_plan, String, required: false, description: 'Plan from which to get billable members.'
+              end
 
         field :dora,
               ::Types::DoraType,
@@ -105,6 +107,10 @@ module EE
               description: 'Compliance violations reported on merge requests merged within the group.' \
                            ' Available only when feature flag `compliance_violations_graphql_type` is enabled. This flag is disabled by default, because the feature is experimental and is subject to change without notice.',
               resolver: ::Resolvers::ComplianceManagement::MergeRequests::ComplianceViolationResolver
+
+        def billable_members_count(requested_hosted_plan: nil)
+          object.billable_members_count(requested_hosted_plan)
+        end
       end
     end
   end

ee/spec/graphql/ee/types/group_type_spec.rb

@@ -66,6 +66,73 @@ RSpec.describe GitlabSchema.types['Group'] do
     end
   end
 
+  describe 'billable members count' do
+    let_it_be(:group) { create(:group) }
+    let_it_be(:project) { create(:project, namespace: group) }
+    let_it_be(:user1) { create(:user) }
+    let_it_be(:user2) { create(:user) }
+    let_it_be(:user3) { create(:user) }
+    let_it_be(:user4) { create(:user) }
+
+    before do
+      group.add_developer(user1)
+      group.add_guest(user2)
+      project.add_developer(user3)
+      project.add_guest(user4)
+    end
+
+    it "returns billable users count including guests when no plan is provided" do
+      query = <<~GQL
+        query {
+          group(fullPath: "#{group.full_path}") {
+            id,
+            billableMembersCount
+          }
+        }
+      GQL
+
+      result = GitlabSchema.execute(query, context: { current_user: user1 }).as_json
+
+      billable_members_count = result.dig('data', 'group', 'billableMembersCount')
+
+      expect(billable_members_count).to eq(4)
+    end
+
+    it "returns billable users count including guests when a plan that should include guests is provided" do
+      query = <<~GQL
+        query {
+          group(fullPath: "#{group.full_path}") {
+            id,
+            billableMembersCount(requestedHostedPlan: "#{::Plan::SILVER}")
+          }
+        }
+      GQL
+
+      result = GitlabSchema.execute(query, context: { current_user: user1 }).as_json
+
+      billable_members_count = result.dig('data', 'group', 'billableMembersCount')
+
+      expect(billable_members_count).to eq(4)
+    end
+
+    it "returns billable users count excluding guests when a plan that should exclude guests is provided" do
+      query = <<~GQL
+        query {
+          group(fullPath: "#{group.full_path}") {
+            id,
+            billableMembersCount(requestedHostedPlan: "#{::Plan::ULTIMATE}")
+          }
+        }
+      GQL
+
+      result = GitlabSchema.execute(query, context: { current_user: user1 }).as_json
+
+      billable_members_count = result.dig('data', 'group', 'billableMembersCount')
+
+      expect(billable_members_count).to eq(2)
+    end
+  end
+
   describe 'dora field' do
     subject { described_class.fields['dora'] }