Expose peer certificates in round tripper response (#3110)

conformance/utils/roundtripper/roundtripper.go

@@ -99,11 +99,12 @@ type RedirectRequest struct {
 
 // CapturedResponse contains response metadata.
 type CapturedResponse struct {
-	StatusCode      int
-	ContentLength   int64
-	Protocol        string
-	Headers         map[string][]string
-	RedirectRequest *RedirectRequest
+	StatusCode       int
+	ContentLength    int64
+	Protocol         string
+	Headers          map[string][]string
+	RedirectRequest  *RedirectRequest
+	PeerCertificates []*x509.Certificate
 }
 
 // DefaultRoundTripper is the default implementation of a RoundTripper. It will
@@ -258,6 +259,10 @@ func (d *DefaultRoundTripper) defaultRoundTrip(request Request, transport http.R
 		Headers:       resp.Header,
 	}
 
+	if resp.TLS != nil {
+		cRes.PeerCertificates = resp.TLS.PeerCertificates
+	}
+
 	if IsRedirect(resp.StatusCode) {
 		redirectURL, err := resp.Location()
 		if err != nil {