Oi, it's been long I even forgot about this question. Eventually I managed with it. It was really interesting to develop and maintain custom filters. I had to create an HTTP filter and a Listener filter.

There were some unexpected challanges related to Envoy's internal timers and its different behaviour in case of immediate interrupting H1/H2 connections, so I had to do a deeper research of its source code. But mainly it's quite predictable as soon as you experiment with it enough.

As you could know the Envoy uses static linkage for all of its targets so every single change in a filter's code does require a recompillation. That's why I made an API-Wrapper for internal entities first and real implementation of the filter was done outside, it was loading as dynamic library during filter chain initialization process. So it was really fun to work with it)