Reapply single-PID adapation for metricbeat's system/process, revert …

…update to `go-systemd` (#39730)

* Reapply "let the system/process metricset monitor a single PID (#39620)" (#39714)

This reverts commit 05c5957.

* reapply single-pid changes, tinker with go-systemd dep

* fix deps

metricbeat/module/system/process/config.go

@@ -32,6 +32,8 @@ type Config struct {
 	IncludeCPUTicks bool                     `config:"process.include_cpu_ticks"`
 	IncludePerCPU   bool                     `config:"process.include_per_cpu"`
 	CPUTicks        *bool                    `config:"cpu_ticks"` // Deprecated
+	// Pid, if set, will override the `processes` config, and only monitor a single process.
+	Pid int `config:"process.pid"`
 }
 
 // Validate checks for depricated config options

metricbeat/module/system/process/process.go

@@ -45,8 +45,8 @@ func init() {
 type MetricSet struct {
 	mb.BaseMetricSet
 	stats  *process.Stats
-	cgroup *cgroup.Reader
 	perCPU bool
+	setpid int
 }
 
 // New creates and returns a new MetricSet.
@@ -65,6 +65,10 @@ func New(base mb.BaseMetricSet) (mb.MetricSet, error) {
 		}
 	}
 
+	if config.Pid != 0 && config.Procs[0] != ".*" {
+		logp.L().Warnf("`process.pid` set to %d, but `processes` is set to a non-default value. Metricset will only report metrics for pid %d", config.Pid, config.Pid)
+	}
+
 	m := &MetricSet{
 		BaseMetricSet: base,
 		stats: &process.Stats{
@@ -83,6 +87,8 @@ func New(base mb.BaseMetricSet) (mb.MetricSet, error) {
 		perCPU: config.IncludePerCPU,
 	}
 
+	m.setpid = config.Pid
+
 	// If hostfs is set, we may not want to force the hierarchy override, as the user could be expecting a custom path.
 	if !sys.IsSet() {
 		override, isset := os.LookupEnv("LIBBEAT_MONITORING_CGROUPS_HIERARCHY_OVERRIDE")
@@ -101,19 +107,32 @@ func New(base mb.BaseMetricSet) (mb.MetricSet, error) {
 // Fetch fetches metrics for all processes. It iterates over each PID and
 // collects process metadata, CPU metrics, and memory metrics.
 func (m *MetricSet) Fetch(r mb.ReporterV2) error {
-	procs, roots, err := m.stats.Get()
-	if err != nil {
-		return fmt.Errorf("process stats: %w", err)
-	}
 
-	for evtI := range procs {
-		isOpen := r.Event(mb.Event{
-			MetricSetFields: procs[evtI],
-			RootFields:      roots[evtI],
-		})
-		if !isOpen {
-			return nil
+	// monitor either a single PID, or the configured set of processes.
+	if m.setpid == 0 {
+		procs, roots, err := m.stats.Get()
+		if err != nil {
+			return fmt.Errorf("process stats: %w", err)
+		}
+
+		for evtI := range procs {
+			isOpen := r.Event(mb.Event{
+				MetricSetFields: procs[evtI],
+				RootFields:      roots[evtI],
+			})
+			if !isOpen {
+				return nil
+			}
 		}
+	} else {
+		proc, root, err := m.stats.GetOneRootEvent(m.setpid)
+		if err != nil {
+			return fmt.Errorf("error fetching pid %d: %w", m.setpid, err)
+		}
+		r.Event(mb.Event{
+			MetricSetFields: proc,
+			RootFields:      root,
+		})
 	}
 
 	return nil

metricbeat/module/system/process/process_test.go

@@ -20,6 +20,7 @@
 package process
 
 import (
+	"os"
 	"testing"
 	"time"
 
@@ -50,6 +51,20 @@ func TestFetch(t *testing.T) {
 		events[0].BeatEvent("system", "process").Fields.StringToPrint())
 }
 
+func TestFetchSinglePid(t *testing.T) {
+	logp.DevelopmentSetup()
+
+	cfg := getConfig()
+	cfg["process.pid"] = os.Getpid()
+
+	f := mbtest.NewReportingMetricSetV2Error(t, cfg)
+	events, errs := mbtest.ReportingFetchV2Error(f)
+	assert.Empty(t, errs)
+	assert.NotEmpty(t, events)
+	assert.Equal(t, os.Getpid(), events[0].RootFields["process"].(map[string]interface{})["pid"])
+	assert.NotEmpty(t, events[0].MetricSetFields["cpu"])
+}
+
 func TestData(t *testing.T) {
 	f := mbtest.NewReportingMetricSetV2Error(t, getConfig())