use istio client-go library instead of knative (#1661)

use istio client-go library instead of knative
bump kubernetes dependency version
change code coverage to codecov

vendor/github.com/prometheus/client_golang/api/client.go

@@ -11,8 +11,6 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
-// +build go1.7
-
 // Package api provides clients for the HTTP APIs.
 package api
 
@@ -60,6 +58,28 @@ type Client interface {
 	Do(context.Context, *http.Request) (*http.Response, []byte, error)
 }
 
+// DoGetFallback will attempt to do the request as-is, and on a 405 it will fallback to a GET request.
+func DoGetFallback(c Client, ctx context.Context, u *url.URL, args url.Values) (*http.Response, []byte, error) {
+	req, err := http.NewRequest(http.MethodPost, u.String(), strings.NewReader(args.Encode()))
+	if err != nil {
+		return nil, nil, err
+	}
+	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+
+	resp, body, err := c.Do(ctx, req)
+	if resp != nil && resp.StatusCode == http.StatusMethodNotAllowed {
+		u.RawQuery = args.Encode()
+		req, err = http.NewRequest(http.MethodGet, u.String(), nil)
+		if err != nil {
+			return nil, nil, err
+		}
+
+	} else {
+		return resp, body, err
+	}
+	return c.Do(ctx, req)
+}
+
 // NewClient returns a new Client.
 //
 // It is safe to use the returned Client from multiple goroutines.
@@ -119,8 +139,8 @@ func (c *httpClient) Do(ctx context.Context, req *http.Request) (*http.Response,
 
 	select {
 	case <-ctx.Done():
-		err = resp.Body.Close()
 		<-done
+		err = resp.Body.Close()
 		if err == nil {
 			err = ctx.Err()
 		}