upstream: only prefetching if the upstream is healthy (#12758)

Risk Level: low (only affects hidden prefetching)
Testing: new unit tests
Docs Changes: n/a
Release Notes: n/a

Signed-off-by: Alyssa Wilk <alyssar@chromium.org>

Mirrored from https://github.com/envoyproxy/envoy @ f4fd39dd13da7d67b96d8a48cc6242c1b35833ff

envoy/config/cluster/v3/cluster.proto

@@ -578,7 +578,8 @@ message Cluster {
   // [#not-implemented-hide:]
   message PrefetchPolicy {
     // Indicates how many many streams (rounded up) can be anticipated per-upstream for each
-    // stream, useful for high-QPS or latency-sensitive services.
+    // stream, useful for high-QPS or latency-sensitive services. Prefetching will only be done if
+    // the upstream is healthy.
     //
     // For example if this is 2, for an incoming HTTP/1.1 stream, 2 connections will be
     // established, one for the new incoming stream, and one for a presumed follow-up stream. For

envoy/config/cluster/v4alpha/cluster.proto

@@ -585,7 +585,8 @@ message Cluster {
         "envoy.config.cluster.v3.Cluster.PrefetchPolicy";
 
     // Indicates how many many streams (rounded up) can be anticipated per-upstream for each
-    // stream, useful for high-QPS or latency-sensitive services.
+    // stream, useful for high-QPS or latency-sensitive services. Prefetching will only be done if
+    // the upstream is healthy.
     //
     // For example if this is 2, for an incoming HTTP/1.1 stream, 2 connections will be
     // established, one for the new incoming stream, and one for a presumed follow-up stream. For