Closing all idle connections in docker input plugin (#9243)

This prevents error "too many open files" in most cases

plugins/inputs/docker/client.go

@@ -23,6 +23,7 @@ type Client interface {
 	ServiceList(ctx context.Context, options types.ServiceListOptions) ([]swarm.Service, error)
 	TaskList(ctx context.Context, options types.TaskListOptions) ([]swarm.Task, error)
 	NodeList(ctx context.Context, options types.NodeListOptions) ([]swarm.Node, error)
+	Close() error
 }
 
 func NewEnvClient() (Client, error) {
@@ -76,3 +77,6 @@ func (c *SocketClient) TaskList(ctx context.Context, options types.TaskListOptio
 func (c *SocketClient) NodeList(ctx context.Context, options types.NodeListOptions) ([]swarm.Node, error) {
 	return c.client.NodeList(ctx, options)
 }
+func (c *SocketClient) Close() error {
+	return c.client.Close()
+}

plugins/inputs/docker/docker.go

@@ -213,6 +213,9 @@ func (d *Docker) Gather(acc telegraf.Accumulator) error {
 		d.client = c
 	}
 
+	// Close any idle connections in the end of gathering
+	defer d.client.Close()
+
 	// Create label filters if not already created
 	if !d.filtersCreated {
 		err := d.createLabelFilters()

plugins/inputs/docker/docker_test.go

@@ -26,6 +26,7 @@ type MockClient struct {
 	ServiceListF      func(ctx context.Context, options types.ServiceListOptions) ([]swarm.Service, error)
 	TaskListF         func(ctx context.Context, options types.TaskListOptions) ([]swarm.Task, error)
 	NodeListF         func(ctx context.Context, options types.NodeListOptions) ([]swarm.Node, error)
+	CloseF            func() error
 }
 
 func (c *MockClient) Info(ctx context.Context) (types.Info, error) {
@@ -75,6 +76,10 @@ func (c *MockClient) NodeList(
 	return c.NodeListF(ctx, options)
 }
 
+func (c *MockClient) Close() error {
+	return c.CloseF()
+}
+
 var baseClient = MockClient{
 	InfoF: func(context.Context) (types.Info, error) {
 		return info, nil
@@ -97,6 +102,9 @@ var baseClient = MockClient{
 	NodeListF: func(context.Context, types.NodeListOptions) ([]swarm.Node, error) {
 		return NodeList, nil
 	},
+	CloseF: func() error {
+		return nil
+	},
 }
 
 func newClient(_ string, _ *tls.Config) (Client, error) {
@@ -279,6 +287,9 @@ func TestDocker_WindowsMemoryContainerStats(t *testing.T) {
 				NodeListF: func(context.Context, types.NodeListOptions) ([]swarm.Node, error) {
 					return NodeList, nil
 				},
+				CloseF: func() error {
+					return nil
+				},
 			}, nil
 		},
 	}