Add configuration for MCP router filter

[yanavlasov]

Extension for abstracting multiple MPC servers as one. Capabilities, tools and resources of remote MCP servers are combined and presented as a single MCP server to the client. This allows a unified policy to be applied to multiple remote servers.

Part of #39174

Risk Level: none
Testing: n/a
Docs Changes: yes
Release Notes: n/a
Platform Specific Features: n/a

[repokitteh-read-only]

CC @envoyproxy/api-shepherds: Your approval is needed for changes made to (api/envoy/|docs/root/api-docs/).
envoyproxy/api-shepherds assignee is @markdroth
CC @envoyproxy/api-watchers: FYI only for changes made to (api/envoy/|docs/root/api-docs/).

🐱

Caused by: #42275 was opened by yanavlasov.

see: more, trace.

[botengyao]

Thanks for adding this proto!

/wait

[botengyao]

Will adding a top layer for backends be better in term of extensionality? Something like the following:

message McpRouter {
  repeated McpBackend backends = 1 [(validate.rules).repeated = {min_items: 1}];
}

message McpBackend {
  string name = 1;
  
  oneof target {
    // Static cluster reference
    string cluster = 2;
    
    // Dynamic URI
    config.core.v3.HttpUri = 3;  // e.g. "https://api.github.com/mcp"
  }
  
  other attributes like prefix ..
}

[yanavlasov]

Sounds good. Note that HttpUri already has cluster in it.

[yanavlasov]

I have updated the proto. Keeping it to the minimum for now to move faster.

I did not want to add the requirement to have minimum 1 backend. It is possible that control plane may have to provision 0 backends while it is in some transitional state. Envoy can respond with 503 if it does not have any MCP servers.

[repokitteh-read-only]

CC @envoyproxy/coverage-shephards: FYI only for changes made to (test/coverage.yaml).
envoyproxy/coverage-shephards assignee is @RyanTheOptimist

🐱

Caused by: #42275 was synchronize by yanavlasov.

see: more, trace.

[yanavlasov]

@RyanTheOptimist we are splitting the work on this among multiple people and need to bypass coverage to get boilerplate committed. I will adjust coverage up as tests and filter code are added.