util/clientmetric: switch to TestHooks struct for test-only functions (#4632)

Followup to 7966aed1e0 to pick up
review feedback that was accidentally left out.

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

util/clientmetric/clientmetric.go

@@ -255,10 +255,6 @@ func EncodeLogTailMetricsDelta() string {
 	return enc.buf.String()
 }
 
-func ResetLastDeltaForTest() {
-	lastDelta = time.Time{}
-}
-
 var deltaPool = &sync.Pool{
 	New: func() any {
 		return new(deltaEncBuf)
@@ -308,3 +304,11 @@ func (b *deltaEncBuf) writeHexVarint(v int64) {
 	hex.Encode(hexBuf, b.scratch[:n])
 	b.buf.Write(hexBuf)
 }
+
+var TestHooks testHooks
+
+type testHooks struct{}
+
+func (testHooks) ResetLastDelta() {
+	lastDelta = time.Time{}
+}