diff --git a/util/clientmetric/clientmetric_test.go b/util/clientmetric/clientmetric_test.go
new file mode 100644
index 000000000..0a49b0adc
--- /dev/null
+++ b/util/clientmetric/clientmetric_test.go
@@ -0,0 +1,75 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package clientmetric
+
+import (
+	"testing"
+	"time"
+)
+
+func TestDeltaEncBuf(t *testing.T) {
+	var enc deltaEncBuf
+	enc.writeName("one_one")
+	enc.writeValue(1, 1)
+	enc.writeName("two_zero")
+	enc.writeValue(2, 0)
+
+	enc.writeDelta(1, 63)
+	enc.writeDelta(2, 64)
+	enc.writeDelta(1, -65)
+	enc.writeDelta(2, -64)
+
+	got := enc.buf.String()
+	const want = "N0eone_oneS0202N10two_zeroS0400I027eI048001I028101I047f"
+	if got != want {
+		t.Errorf("error\n got %q\nwant %q\n", got, want)
+	}
+}
+
+func clearMetrics() {
+	mu.Lock()
+	defer mu.Unlock()
+	metrics = map[string]*Metric{}
+	numWireID = 0
+	lastDelta = time.Time{}
+	sorted = nil
+	lastLogVal = nil
+	unsorted = nil
+}
+
+func advanceTime() {
+	mu.Lock()
+	defer mu.Unlock()
+	lastDelta = time.Time{}
+}
+
+func TestEncodeLogTailMetricsDelta(t *testing.T) {
+	clearMetrics()
+
+	c1 := NewCounter("foo")
+	c2 := NewCounter("bar")
+	c1.Add(123)
+	if got, want := EncodeLogTailMetricsDelta(), "N06fooS02f601"; got != want {
+		t.Errorf("first = %q; want %q", got, want)
+	}
+
+	c2.Add(456)
+	advanceTime()
+	if got, want := EncodeLogTailMetricsDelta(), "N06barS049007"; got != want {
+		t.Errorf("second = %q; want %q", got, want)
+	}
+
+	advanceTime()
+	if got, want := EncodeLogTailMetricsDelta(), ""; got != want {
+		t.Errorf("with no changes = %q; want %q", got, want)
+	}
+
+	c1.Add(1)
+	c2.Add(2)
+	advanceTime()
+	if got, want := EncodeLogTailMetricsDelta(), "I0202I0404"; got != want {
+		t.Errorf("with increments = %q; want %q", got, want)
+	}
+}