Make over provisioning factor configurable (#4003)

Description:
Add over-provisioning-factor to EDS proto.
Add a over_provisioning_factor() method for HostSet class.

Risk Level: Medium
Testing: Unit tests.
Docs Changes: inline
#3222

Signed-off-by: Xin Zhuang <stevenzzz@google.com>

Mirrored from https://github.com/envoyproxy/envoy @ 8c189a5521eb7da6c4976afb095356b549d9c1ca

envoy/api/v2/eds.proto

@@ -12,6 +12,7 @@ import "google/api/annotations.proto";
 
 import "validate/validate.proto";
 import "gogoproto/gogo.proto";
+import "google/protobuf/wrappers.proto";
 
 option (gogoproto.equal_all) = true;
 
@@ -80,6 +81,17 @@ message ClusterLoadAssignment {
     //    "lb"_drop = 20%  // 50% of the remaining 'actual' load, which is 40%.
     //    actual_outgoing_load = 20% // remaining after applying all categories.
     repeated DropOverload drop_overloads = 2;
+
+    // Priority levels and localities are considered overprovisioned with this
+    // factor (in percentage). This means that we don't consider a priority
+    // level or locality unhealthy until the percentage of healthy hosts
+    // multiplied by the overprovisioning factor drops below 100.
+    // With the default value 140(1.4), Envoy doesn't consider a priority level
+    // or a locality unhealthy until their percentage of healthy hosts drops
+    // below 72%.
+    // Read more at :ref:`priority levels <arch_overview_load_balancing_priority_levels>` and
+    // :ref:`localities <arch_overview_load_balancing_locality_weighted_lb>`.
+    google.protobuf.UInt32Value overprovisioning_factor = 3 [(validate.rules).uint32.gt = 0];
   }
 
   // Load balancing policy settings.