metrics: Document NewProcessLabels function

Hopefully this comment makes it clearer how the labels filtering happens.

Signed-off-by: Anna Kapuscinska <[email protected]>

pkg/metrics/filteredlabels.go

@@ -19,6 +19,10 @@ type ProcessLabels struct {
 	binary    string
 }
 
+// NewProcessLabels creates a new ProcessLabels struct with the global labels
+// filter applied. To have a metric respect the labels filter, we have to:
+//  1. Define a granular metric with ProcessLabels type parameter (see granularmetric.go).
+//  2. When calling WithLabelValues, pass a ProcessLabels struct created with NewProcessLabels.
 func NewProcessLabels(namespace, workload, pod, binary string) *ProcessLabels {
 	if !option.Config.MetricsLabelFilter["namespace"] {
 		namespace = ""