dns: adjust DNS refresh rate respecting to record TTL (#6975)

Signed-off-by: Yan Xue <yxyan@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ e0e7628c3bc4227245f15c4f047ddad04912351c

envoy/api/v2/cds.proto

@@ -51,7 +51,7 @@ service ClusterDiscoveryService {
 // [#protodoc-title: Clusters]
 
 // Configuration for a single upstream cluster.
-// [#comment:next free field: 39]
+// [#comment:next free field: 40]
 message Cluster {
   // Supplies the name of the cluster which must be unique across all clusters.
   // The cluster name is used when emitting
@@ -274,6 +274,11 @@ message Cluster {
   google.protobuf.Duration dns_refresh_rate = 16
       [(validate.rules).duration.gt = {}, (gogoproto.stdduration) = true];
 
+  // Optional configuration for setting cluster's DNS refresh rate. If the value is set to true,
+  // cluster's DNS refresh rate will be set to resource record's TTL which comes from DNS
+  // resolution.
+  bool respect_dns_ttl = 39;
+
   // When V4_ONLY is selected, the DNS resolver will only perform a lookup for
   // addresses in the IPv4 family. If V6_ONLY is selected, the DNS resolver will
   // only perform a lookup for addresses in the IPv6 family. If AUTO is