[system/process]: mark module as healthy if metrics are partially fi…

…lled for single process as well. (#40924)

* fix: mark module as healthy if metrics are partially filled for single processes

* chore: fix lint

* chore: wrap the error and return

* chore: fix lint

---------

Co-authored-by: subham sarkar <subham.sarkar@elastic.co>

metricbeat/module/system/process/process.go

@@ -57,7 +57,10 @@ func New(base mb.BaseMetricSet) (mb.MetricSet, error) {
 		return nil, err
 	}
 
-	sys := base.Module().(resolve.Resolver)
+	sys, ok := base.Module().(resolve.Resolver)
+	if !ok {
+		return nil, fmt.Errorf("resolver cannot be cast from the module")
+	}
 	enableCgroups := false
 	if runtime.GOOS == "linux" {
 		if config.Cgroups == nil || *config.Cgroups {
@@ -131,14 +134,17 @@ func (m *MetricSet) Fetch(r mb.ReporterV2) error {
 		return err
 	} else {
 		proc, root, err := m.stats.GetOneRootEvent(m.setpid)
-		if err != nil {
+		if err != nil && !errors.Is(err, process.NonFatalErr{}) {
+			// return only if the error is fatal in nature
 			return fmt.Errorf("error fetching pid %d: %w", m.setpid, err)
+		} else if (err != nil && errors.Is(err, process.NonFatalErr{})) {
+			err = mb.PartialMetricsError{Err: err}
 		}
+		// if error is non-fatal, emit partial metrics.
 		r.Event(mb.Event{
 			MetricSetFields: proc,
 			RootFields:      root,
 		})
+		return err
 	}
-
-	return nil
 }