Set or forward X-Request-ID header

Ensures that `X-Request-ID` is defaulted to a random value, if no
`X-Request-ID` is present in the request.

* https://stackoverflow.com/questions/25433258/what-is-the-x-request-id-http-header
* https://en.wikipedia.org/wiki/List_of_HTTP_header_fields
* http://nginx.org/en/docs/http/ngx_http_core_module.html
* https://kubernetes.github.io/ingress-nginx/user-guide/nginx-configuration/configmap/#generate-request-id

Author: itskingori

config/http.conf

@@ -30,6 +30,13 @@ http {
     ''      $server_port;
   }
 
+  # If we receive X-Request-ID, pass it through; otherwise, pass along the
+  # request_id generated by nginx
+  map $http_x_request_id $proxy_x_request_id {
+    default $http_x_request_id;
+    ''      $request_id;
+  }
+
   proxy_http_version  1.1;
   proxy_set_header    Connection          "";
   proxy_set_header    Host                $host;
@@ -39,6 +46,7 @@ http {
   proxy_set_header    X-Forwarded-Proto   $proxy_x_forwarded_proto;
   proxy_set_header    X-Forwarded-Ssl     $proxy_x_forwarded_ssl;
   proxy_set_header    X-Forwarded-Port    $proxy_x_forwarded_port;
+  proxy_set_header    X-Request-ID        $proxy_x_request_id;
   proxy_set_header    X-Forwarded-Host    $host;
 
   proxy_read_timeout  600s;