[Docker] Stop returning errors when there are no metric values (#39807)

* Return nil instead or an error if no metrics

The docker metricset returns an error when there are no memory metric
values available.

This condition can happen when there are no running containers on
Docker.

When no containers are running, the metricset returns an error
at every collection, creating noise.

* Add a "docker.memory" logger to write a debug message on no-metrics

metricbeat/module/docker/memory/memory.go

@@ -22,6 +22,8 @@ package memory
 import (
 	"fmt"
 
+	"github.com/elastic/elastic-agent-libs/logp"
+
 	"github.com/docker/docker/client"
 
 	"github.com/elastic/beats/v7/metricbeat/mb"
@@ -41,25 +43,28 @@ type MetricSet struct {
 	memoryService *MemoryService
 	dockerClient  *client.Client
 	dedot         bool
+	logger        *logp.Logger
 }
 
 // New creates a new instance of the docker memory MetricSet.
 func New(base mb.BaseMetricSet) (mb.MetricSet, error) {
+	logger := logp.NewLogger("docker.memory")
 	config := docker.DefaultConfig()
 	if err := base.Module().UnpackConfig(&config); err != nil {
 		return nil, err
 	}
 
-	client, err := docker.NewDockerClient(base.HostData().URI, config)
+	dockerClient, err := docker.NewDockerClient(base.HostData().URI, config)
 	if err != nil {
 		return nil, err
 	}
 
 	return &MetricSet{
 		BaseMetricSet: base,
 		memoryService: &MemoryService{},
-		dockerClient:  client,
+		dockerClient:  dockerClient,
 		dedot:         config.DeDot,
+		logger:        logger,
 	}, nil
 }
 
@@ -72,7 +77,10 @@ func (m *MetricSet) Fetch(r mb.ReporterV2) error {
 
 	memoryStats := m.memoryService.getMemoryStatsList(stats, m.dedot)
 	if len(memoryStats) == 0 {
-		return fmt.Errorf("No memory stats data available")
+		// No memory stats available, probably
+		// because no containers are running.
+		m.logger.Debug("No memory stats data available")
+		return nil
 	}
 	eventsMapping(r, memoryStats)