Feature for probing functions

Introduces a single-flight call to a function's health
endpoint to verify that it is registered with an Istio
sidecar (Envoy) before letting the invocation through.

Results are cached for 5 seconds, before a probe is
required again.

Tested without Istio, with probe_functions environment
variable set to true, I saw a probe execute in the logs.

Fixes: #1721 for Istio users.

Signed-off-by: Alex Ellis (OpenFaaS Ltd) <alex@openfaas.com>

gateway/pkg/middleware/transparent_url_path_transformer_test.go

@@ -0,0 +1,34 @@
+// Copyright (c) OpenFaaS Author(s). All rights reserved.
+// Licensed under the MIT license. See LICENSE file in the project root for full license information.
+
+package middleware
+
+import (
+	"net/http"
+	"testing"
+)
+
+func Test_Transform_DoesntTransformRootPath(t *testing.T) {
+
+	req, _ := http.NewRequest(http.MethodGet, "/", nil)
+	transformer := TransparentURLPathTransformer{}
+	want := req.URL.Path
+	got := transformer.Transform(req)
+
+	if want != got {
+		t.Errorf("want: %s, got: %s", want, got)
+	}
+}
+
+func Test_Transform_DoesntTransformAdditionalPath(t *testing.T) {
+
+	req, _ := http.NewRequest(http.MethodGet, "/employees/", nil)
+	transformer := TransparentURLPathTransformer{}
+	want := req.URL.Path
+	got := transformer.Transform(req)
+
+	if want != got {
+		t.Errorf("want: %s, got: %s", want, got)
+	}
+
+}