inline conditional envoy response

envoy.yaml

static_resources:
  listeners:
  - name: main
    address:
      socket_address:
        address: 0.0.0.0
        port_value: 8000
    filter_chains:
    - filters:
      - name: envoy.filters.network.http_connection_manager
        typed_config:
          "@type": type.googleapis.com/envoy.extensions.filters.network.http_connection_manager.v3.HttpConnectionManager
          stat_prefix: ingress_http
          codec_type: auto
          route_config:
            name: local_route
            virtual_hosts:
            - name: local_service
              domains:
              - "*"
              routes:
              - match:
                  prefix: "/"
                route:
                  cluster: web_service
          http_filters:
          - name: envoy.filters.http.lua
            typed_config:
              "@type": type.googleapis.com/envoy.extensions.filters.http.lua.v3.Lua
              inline_code: |
                local mylibrary = require("lib.mylibrary")

                function envoy_on_request(request_handle)
                  request_handle:headers():add("foo", mylibrary.foobar())
                end
                function envoy_on_response(response_handle)
                  if response_handle:headers():get(":status") == "503" then
                    local content_length = response_handle:body():setBytes("custom error text")
                    response_handle:headers():replace("content-length", content_length)
                    response_handle:headers():replace("content-type", "text/html")
                  end
                  body_size = response_handle:body():length()
                  response_handle:headers():add("response-body-size", tostring(body_size))
                end
          - name: envoy.filters.http.router
            typed_config: {}

  clusters:
  - name: web_service
    connect_timeout: 0.25s
    type: strict_dns # static
    lb_policy: round_robin
    load_assignment:
      cluster_name: web_service
      endpoints:
      - lb_endpoints:
        - endpoint:
            address:
              socket_address:
                address: web_service
                port_value: 80