metrics: Improve doc of FilteredLabels interface

Signed-off-by: Anna Kapuscinska <anna@isovalent.com>

pkg/metrics/doc.go

@@ -40,9 +40,14 @@
 //
 // `ConstrainedLabel` and `UnconstrainedLabel` structs represent metric labels.
 //
-// `FilteredLabels` interface represents configurable labels, passed to metrics
-// via type parameter. The values are always unconstrained. `NilLabels` package
-// variable is a special case of `FilteredLabels` with no labels.
+// `FilteredLabels` interface represents configurable labels. It's intended to
+// be used as a type parameter when defining a granular metric, to add common
+// labels. The idea is that users can configure which of these (potentially
+// high-cardinality) labels are actually exposed - see `CreateProcessLabels` in
+// `pkg/option` for an example. The values of these labels are always
+// unconstrained. `NilLabels` package variable is a special case of
+// `FilteredLabels` with no labels, which is used in convenience wrappers
+// around granular metrics.
 //
 // `GranularCounter[L FilteredLabels]` (and analogous Gauge and Histogram)
 // struct is a wrapper around `prometheus.CounterVec` (Gauge, Histogram) with