util/clientmetric: add gauge_ name prefix when uploading names

The prefix is a signal to tsweb to treat this as a gauge metric when
generating the Prometheus version.

Signed-off-by: Mihai Parparita <mihai@tailscale.com>

util/clientmetric/clientmetric.go

@@ -241,7 +241,7 @@ func EncodeLogTailMetricsDelta() string {
 			m.wireID = numWireID
 		}
 		if m.lastNamed.IsZero() || now.Sub(m.lastNamed) > metricLogNameFrequency {
-			enc.writeName(m.Name())
+			enc.writeName(m.Name(), m.Type())
 			m.lastNamed = now
 			enc.writeValue(m.wireID, val)
 		} else {
@@ -271,9 +271,16 @@ type deltaEncBuf struct {
 // writeName writes a "name" (N) record to the buffer, which notes
 // that the immediately following record's wireID has the provided
 // name.
-func (b *deltaEncBuf) writeName(name string) {
+func (b *deltaEncBuf) writeName(name string, typ Type) {
+	var namePrefix string
+	if typ == TypeGauge {
+		// Add the gauge_ prefix so that tsweb knows that this is a gauge metric
+		// when generating the Prometheus version.
+		namePrefix = "gauge_"
+	}
 	b.buf.WriteByte('N')
-	b.writeHexVarint(int64(len(name)))
+	b.writeHexVarint(int64(len(namePrefix) + len(name)))
+	b.buf.WriteString(namePrefix)
 	b.buf.WriteString(name)
 }
 

util/clientmetric/clientmetric_test.go

@@ -11,9 +11,9 @@ import (
 
 func TestDeltaEncBuf(t *testing.T) {
 	var enc deltaEncBuf
-	enc.writeName("one_one")
+	enc.writeName("one_one", TypeCounter)
 	enc.writeValue(1, 1)
-	enc.writeName("two_zero")
+	enc.writeName("two_zero", TypeGauge)
 	enc.writeValue(2, 0)
 
 	enc.writeDelta(1, 63)
@@ -22,7 +22,7 @@ func TestDeltaEncBuf(t *testing.T) {
 	enc.writeDelta(2, -64)
 
 	got := enc.buf.String()
-	const want = "N0eone_oneS0202N10two_zeroS0400I027eI048001I028101I047f"
+	const want = "N0eone_oneS0202N1cgauge_two_zeroS0400I027eI048001I028101I047f"
 	if got != want {
 		t.Errorf("error\n got %q\nwant %q\n", got, want)
 	}
@@ -49,7 +49,7 @@ func TestEncodeLogTailMetricsDelta(t *testing.T) {
 	clearMetrics()
 
 	c1 := NewCounter("foo")
-	c2 := NewCounter("bar")
+	c2 := NewGauge("bar")
 	c1.Add(123)
 	if got, want := EncodeLogTailMetricsDelta(), "N06fooS02f601"; got != want {
 		t.Errorf("first = %q; want %q", got, want)
@@ -57,7 +57,7 @@ func TestEncodeLogTailMetricsDelta(t *testing.T) {
 
 	c2.Add(456)
 	advanceTime()
-	if got, want := EncodeLogTailMetricsDelta(), "N06barS049007"; got != want {
+	if got, want := EncodeLogTailMetricsDelta(), "N12gauge_barS049007"; got != want {
 		t.Errorf("second = %q; want %q", got, want)
 	}