add slot height metric + SlotHeight type

pkg/monitoring/metrics/metrics.go

@@ -64,6 +64,14 @@ func init() {
 		},
 		feedLabels,
 	)
+
+	// init gauge for slot height
+	gauges[types.SlotHeightMetric] = promauto.NewGaugeVec(
+		prometheus.GaugeOpts{
+			Name: types.SlotHeightMetric,
+		},
+		[]string{"chain", "url"},
+	)
 }
 
 type FeedInput struct {

pkg/monitoring/metrics/slotheight.go

@@ -0,0 +1,37 @@
+package metrics
+
+import (
+	"github.com/prometheus/client_golang/prometheus"
+	commonMonitoring "github.com/smartcontractkit/chainlink-common/pkg/monitoring"
+
+	"github.com/smartcontractkit/chainlink-solana/pkg/monitoring/types"
+)
+
+//go:generate mockery --name SlotHeight --output ./mocks/
+
+type SlotHeight interface {
+	Set(slot types.SlotHeight, chain, url string)
+	Cleanup()
+}
+
+var _ SlotHeight = (*slotHeight)(nil)
+
+type slotHeight struct {
+	simpleGauge
+	labels prometheus.Labels
+}
+
+func NewSlotHeight(log commonMonitoring.Logger) *slotHeight {
+	return &slotHeight{
+		simpleGauge: newSimpleGauge(log, types.SlotHeightMetric),
+	}
+}
+
+func (sh *slotHeight) Set(slot types.SlotHeight, chain, url string) {
+	sh.labels = prometheus.Labels{"chain": chain, "url": url}
+	sh.set(float64(slot), sh.labels)
+}
+
+func (sh *slotHeight) Cleanup() {
+	sh.delete(sh.labels)
+}

pkg/monitoring/metrics/slotheight_test.go

@@ -0,0 +1,38 @@
+package metrics
+
+import (
+	"testing"
+
+	"github.com/prometheus/client_golang/prometheus"
+	"github.com/prometheus/client_golang/prometheus/testutil"
+	"github.com/stretchr/testify/assert"
+	"github.com/stretchr/testify/require"
+
+	"github.com/smartcontractkit/chainlink-common/pkg/logger"
+
+	"github.com/smartcontractkit/chainlink-solana/pkg/monitoring/types"
+)
+
+func TestSlotHeight(t *testing.T) {
+	lgr := logger.Test(t)
+	m := NewSlotHeight(lgr)
+
+	// fetching gauges
+	g, ok := gauges[types.SlotHeightMetric]
+	require.True(t, ok)
+
+	v := 100
+
+	// set gauge
+	assert.NotPanics(t, func() { m.Set(types.SlotHeight(v), t.Name(), t.Name()+"_url") })
+	promBal := testutil.ToFloat64(g.With(prometheus.Labels{
+		"chain": t.Name(),
+		"url":   t.Name() + "_url",
+	}))
+	assert.Equal(t, float64(v), promBal)
+
+	// cleanup gauges
+	assert.Equal(t, 1, testutil.CollectAndCount(g))
+	assert.NotPanics(t, func() { m.Cleanup() })
+	assert.Equal(t, 0, testutil.CollectAndCount(g))
+}

pkg/monitoring/source_slotheight.go

@@ -39,5 +39,6 @@ type slotHeightSource struct {
 }
 
 func (t *slotHeightSource) Fetch(ctx context.Context) (interface{}, error) {
-	return t.client.GetSlot(ctx) // TODO: wrap the type to make it clear which type it is?
+	slot, err := t.client.GetSlot(ctx)
+	return types.SlotHeight(slot), err
 }

pkg/monitoring/types/types.go

@@ -1,5 +1,11 @@
 package types
 
+// types.go contains simple types, more complex types should have a separate file
 const (
-	SlotHeightType = "slot_height"
+	SlotHeightType   = "slot_height"
+	SlotHeightMetric = "sol_" + SlotHeightType
 )
+
+// SlotHeight type wraps the uint64 type returned by the RPC call
+// this helps to delineate types when sending to the exporter
+type SlotHeight uint64