API v2 HeaderValueOption is used for modifying request and response headers in the RouteConfiguration, VirtualHosts, and RouteAction messages. The v2 documentation indicates that the default behavior is to replace headers with the given value. In practice, Envoy appends values to existing headers and the replace behavior cannot be enabled.
HeaderValueOption definition
API v2 HeaderValueOption is used for modifying request and response headers in the RouteConfiguration, VirtualHosts, and RouteAction messages. The v2 documentation indicates that the default behavior is to replace headers with the given value. In practice, Envoy appends values to existing headers and the replace behavior cannot be enabled.
HeaderValueOption definition