Reset metric observed by TestProxyHandler for repeatable test runs.

This test expects x509_missing_san_total to be 1. When run with
-test.count greater than 1, that expectation is unmet for all but the
first test run.

staging/src/k8s.io/kube-aggregator/pkg/apiserver/handler_proxy_test.go

@@ -301,6 +301,7 @@ func TestProxyHandler(t *testing.T) {
 	target := &targetHTTPHandler{}
 	for name, tc := range tests {
 		target.Reset()
+		legacyregistry.Reset()
 
 		func() {
 			targetServer := httptest.NewUnstartedServer(target)