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/types/retry.go

@@ -0,0 +1,25 @@
+package types
+
+import (
+	"log"
+	"time"
+)
+
+type routine func(attempt int) error
+
+func Retry(r routine, label string, attempts int, interval time.Duration) error {
+	var err error
+
+	for i := 0; i < attempts; i++ {
+		res := r(i)
+		if res != nil {
+			err = res
+			log.Printf("[%s]: %d/%d, error: %s\n", label, i, attempts, res)
+		} else {
+			err = nil
+			break
+		}
+		time.Sleep(interval)
+	}
+	return err
+}