Add timestamp to HealthCheckEvent definition (#4119)

This PR adds timestamp field to the HealthCheckEvent message to allow
having it rendered inside the JSON serialized log of a health check
event.

Signed-off-by: Dhi Aurrahman <dio@tetrate.io>

Mirrored from https://github.com/envoyproxy/envoy @ 8d0680feb074999c18998930b7f5f261f8f4a7a0

envoy/data/core/v2alpha/health_check_event.proto

@@ -6,6 +6,7 @@ import "envoy/api/v2/core/address.proto";
 import "envoy/api/v2/core/base.proto";
 
 import "google/protobuf/duration.proto";
+import "google/protobuf/timestamp.proto";
 import "google/protobuf/wrappers.proto";
 
 import "validate/validate.proto";
@@ -30,6 +31,9 @@ message HealthCheckEvent {
     // Host addition.
     HealthCheckAddHealthy add_healthy_event = 5;
   }
+
+  // Timestamp for event.
+  google.protobuf.Timestamp timestamp = 6 [(gogoproto.stdtime) = true];
 }
 
 enum HealthCheckFailureType {