upstream: degraded stats and hc event logs (#5530)

Signed-off-by: Snow Pettersen <snowp@squareup.com>

Mirrored from https://github.com/envoyproxy/envoy @ e6a42cfb80524b32fc4c2354f73b4c3882370bb2

envoy/data/core/v2alpha/health_check_event.proto

@@ -32,6 +32,12 @@ message HealthCheckEvent {
 
     // Host failure.
     HealthCheckFailure health_check_failure_event = 7;
+
+    // Healthy host became degraded.
+    DegradedHealthyHost degraded_healthy_host = 8;
+
+    // A degraded host returned to being healthy.
+    NoLongerDegradedHost no_longer_degraded_host = 9;
   }
 
   // Timestamp for event.
@@ -69,3 +75,9 @@ message HealthCheckFailure {
   // Whether this event is the result of the first ever health check on a host.
   bool first_check = 2;
 }
+
+message DegradedHealthyHost {
+}
+
+message NoLongerDegradedHost {
+}