Implement simple basic auth for the exporter

So this can be quickly thrown into public projects until better
methods of authentications are available

registry.go

@@ -9,6 +9,7 @@ the LICENSE file.
 package registry
 
 import (
+	"encoding/base64"
 	"encoding/json"
 	"github.com/matttproud/golang_instrumentation/maths"
 	"github.com/matttproud/golang_instrumentation/metrics"
@@ -117,6 +118,31 @@ func (r *Registry) Register(name string, metric metrics.Metric) {
 	}
 }
 
+func (register *Registry) YieldProtectedExporter(username, password string) http.HandlerFunc {
+	exporter := register.YieldExporter()
+
+	return http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		authenticated := false
+
+		if auth := r.Header.Get("Authorization"); auth != "" {
+			base64Encoded := strings.SplitAfter(auth, " ")[1]
+			decoded, err := base64.URLEncoding.DecodeString(base64Encoded)
+			if err == nil {
+				usernamePassword := strings.Split(string(decoded), ":")
+				if usernamePassword[0] == username && usernamePassword[1] == password {
+					authenticated = true
+				}
+			}
+		}
+
+		if authenticated {
+			exporter.ServeHTTP(w, r)
+		} else {
+			http.Error(w, "access forbidden", 403)
+		}
+	})
+}
+
 /*
 Create a http.HandlerFunc that is tied to r Registry such that requests
 against it generate a representation of the housed metrics.