Extending Echo filter to match specific metadata

[wtrocki]

Title: Extending Echo filter to match specific metadata

Description:
Extending Echo filter to support metadataMatcher.

Echo filter is now only Network filter. Refactor would be required to enable echo support in HTTP filters.

Echo filter is quite useful for testing complex envoy configurations. Having ability to enable echo per specific matching criteria would be beneficial.
Creating issue to validate idea - I would like to contribute the change.

Relevant Links
https://www.envoyproxy.io/docs/envoy/latest/api-v3/extensions/filters/network/echo/v3/echo.proto

[ravenblackx]

@zuercher maybe?

[zuercher]

My first thought is that if we were to introduce an HTTP echo filter it should be a separate filter. There's already generic code to implement envoy::type::matcher::v3::MetadataMatcher so I don't think there's going to be a lot of duplication between network and http echo filters that support metadata matching.

I think the first step would be to propose the API changes for the existing echo filter and the new http echo filter and go from there.

cc @htuch @alyssawilk since they nominally own the echo network filter

[htuch]

+1, what @zuercher wrote sounds like a plan.

[wtrocki]

I eventually managed to find much better solution for the problem:
https://www.envoyproxy.io/docs/envoy/latest/configuration/http/http_filters/tap_filter

I think tap filter could be mentioned in the documentation as much better alternative to echo filter.