cds: Configure LRS in CDS response (#7953)

This PR adds fields to CDS that allow for configuring LRS.

Risk Level: Low
Testing: None (but if anything is needed, please let me know)
Docs Changes: Inline with API protos

Signed-off-by: Mark D. Roth <roth@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ ca3056b3aeabcfbe3750d611a8dfe44a3a7de1cf

envoy/api/v2/cds.proto

@@ -48,7 +48,7 @@ service ClusterDiscoveryService {
 // [#protodoc-title: Clusters]
 
 // Configuration for a single upstream cluster.
-// [#comment:next free field: 42]
+// [#comment:next free field: 43]
 message Cluster {
   // Supplies the name of the cluster which must be unique across all clusters.
   // The cluster name is used when emitting
@@ -656,6 +656,22 @@ message Cluster {
   // :ref:`lb_policy<envoy_api_field_Cluster.lb_policy>` field has the value
   // :ref:`LOAD_BALANCING_POLICY_CONFIG<envoy_api_enum_value_Cluster.LbPolicy.LOAD_BALANCING_POLICY_CONFIG>`.
   LoadBalancingPolicy load_balancing_policy = 41;
+
+  // [#not-implemented-hide:]
+  // If present, tells the client where to send load reports via LRS. If not present, the
+  // client will fall back to a client-side default, which may be either (a) don't send any
+  // load reports or (b) send load reports for all clusters to a single default server
+  // (which may be configured in the bootstrap file).
+  //
+  // Note that if multiple clusters point to the same LRS server, the client may choose to
+  // create a separate stream for each cluster or it may choose to coalesce the data for
+  // multiple clusters onto a single stream. Either way, the client must make sure to send
+  // the data for any given cluster on no more than one stream.
+  //
+  // [#next-major-version: In the v3 API, we should consider restructuring this somehow,
+  // maybe by allowing LRS to go on the ADS stream, or maybe by moving some of the negotiation
+  // from the LRS stream here.]
+  core.ConfigSource lrs_server = 42;
 }
 
 // [#not-implemented-hide:] Extensible load balancing policy configuration.