[Feature] Support Client Status Discovery Service (CSDS)

[lidizheng]

Title: [Feature] Support Client Status Discovery Service (CSDS)

Description:

Client Status Discovery Service (CSDS) is a service that exposes xDS config of a given client. It’s commonly used to query control planes for the synced xDS config of a particular sidecar proxy. However, it can also be used to query Envoy for its received xDS configuration.

As a xDS client, Envoy can dump xDS config and expose internal status of each xDS resource (#13121). The xDS config dump will share the same proto definition as admin interface. It will be an alternative way of getting config dump. As for internal status, it describes if the Envoy accepts or rejects or is pending on a specific xDS resource.

Relevant Links:

• For Googler, go/grpc-csds
• CSDS proto definition

CC @htuch for more potential benefits.

[mattklein123]

It seems like we should ultimately converge on one dump/status API. Long term should we deprecate the ad-hoc config dump admin API in Envoy and move to CSDS? cc @dschaller @htuch

[htuch]

Always a fan of moving bits of admin to RPC, but I think you've pointed out in the past that there is a significant class of users that wants a simple curl interface? Are you thinking we might have some way to still curl the CSDS output through admin?

[mattklein123]

I'm suggesting some sharing of protos at minimum, and possibly yes supporting a REST fetch of the API (which is easily supported with gRPC unary requests).