Extending Echo filter to match specific metadata about envoy HOT 4 CLOSED

@zuercher maybe?

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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

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

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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.

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