metrics: Add BPF metrics types

These types are slim helpers intended to be used in custom collectors that read
metrics directly from BPF maps.

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

pkg/metrics/bpfmetric.go

@@ -0,0 +1,47 @@
+// SPDX-License-Identifier: Apache-2.0
+// Copyright Authors of Tetragon
+
+package metrics
+
+import "github.com/prometheus/client_golang/prometheus"
+
+// BPFMetric represents a metric read directly from a BPF map.
+// It's intended to be used in custom collectors. The interface doesn't provide
+// any validation, so it's up to the collector implementer to guarantee the
+// metrics consistency.
+type BPFMetric interface {
+	Desc() *prometheus.Desc
+	MustMetric(value float64, labelValues ...string) prometheus.Metric
+}
+
+type bpfCounter struct {
+	desc *prometheus.Desc
+}
+
+func NewBPFCounter(desc *prometheus.Desc) BPFMetric {
+	return &bpfCounter{desc: desc}
+}
+
+func (c *bpfCounter) Desc() *prometheus.Desc {
+	return c.desc
+}
+
+func (c *bpfCounter) MustMetric(value float64, labelValues ...string) prometheus.Metric {
+	return prometheus.MustNewConstMetric(c.desc, prometheus.CounterValue, value, labelValues...)
+}
+
+type bpfGauge struct {
+	desc *prometheus.Desc
+}
+
+func NewBPFGauge(desc *prometheus.Desc) BPFMetric {
+	return &bpfGauge{desc: desc}
+}
+
+func (g *bpfGauge) Desc() *prometheus.Desc {
+	return g.desc
+}
+
+func (g *bpfGauge) MustMetric(value float64, labelValues ...string) prometheus.Metric {
+	return prometheus.MustNewConstMetric(g.desc, prometheus.GaugeValue, value, labelValues...)
+}