api: add control plane identifier (#5860)

This PR adds API support required for #5770 . Once #5844 lands, I will output it using the text stat.

Risk Level: Low
Testing: N/A

Signed-off-by: Rama Chavali <rama.rao@salesforce.com>

Mirrored from https://github.com/envoyproxy/envoy @ 28fbbaa76026f52412c2550ff78664ca79c35487

envoy/api/v2/core/base.proto

@@ -240,3 +240,11 @@ message RuntimeFractionalPercent {
   // Runtime key for a YAML representation of a FractionalPercent.
   string runtime_key = 2;
 }
+
+// Identifies a specific ControlPlane instance that Envoy is connected to.
+message ControlPlane {
+  // An opaque control plane identifier that uniquely identifies an instance
+  // of control plane. This can be used to identify which control plane instance,
+  // the Envoy is connected to.
+  string identifier = 1;
+}

envoy/api/v2/discovery.proto

@@ -96,6 +96,10 @@ message DiscoveryResponse {
   // DiscoveryRequest bearing the nonce. The nonce is optional and is not
   // required for non-stream based xDS implementations.
   string nonce = 5;
+
+  // [#not-implemented-hide:]
+  // The control plane instance that sent the response.
+  core.ControlPlane control_plane = 6;
 }
 
 // IncrementalDiscoveryRequest and IncrementalDiscoveryResponse are used in a