Fix least request lb not fair (#29873)

* Add new idea for selecting hosts among those not selected yet.

Signed-off-by: Leonardo da Mata <ldamata@spotify.com>

* Change how we choose full table scan

Signed-off-by: Leonardo da Mata <ldamata@spotify.com>

* Remove cout

Signed-off-by: Leonardo da Mata <ldamata@spotify.com>

* Fix Tests for load_balancer_impl_test

Signed-off-by: Leonardo da Mata <ldamata@spotify.com>

* Fix format and make sure full scan happens only when selected or the number of choices is larger than the size.

Signed-off-by: Leonardo da Mata <ldamata@spotify.com>

* Enable new option on extesions api only

Signed-off-by: Leonardo da Mata <ldamata@spotify.com>

* Fix Integration tests.

Signed-off-by: Leonardo da Mata <ldamata@spotify.com>

* Add release notes for full scan in least request LB.

Signed-off-by: Leonardo da Mata <ldamata@spotify.com>

* Fix ref for release note.

Signed-off-by: Leonardo da Mata <ldamata@spotify.com>

* Fix release notes

Signed-off-by: Leonardo da Mata <ldamata@spotify.com>

* Update release note

Signed-off-by: Leonardo da Mata <ldamata@spotify.com>

---------

Signed-off-by: Leonardo da Mata <ldamata@spotify.com>
Signed-off-by: Leonardo da Mata <barroca@gmail.com>
Co-authored-by: Leonardo da Mata <ldamata@spotify.com>

Mirrored from https://github.com/envoyproxy/envoy @ 3ea2bc40590c1a48f26e8297ae55d7a6d08083e9

envoy/extensions/load_balancing_policies/least_request/v3/least_request.proto

@@ -22,6 +22,7 @@ option (udpa.annotations.file_status).package_version_status = ACTIVE;
 // This configuration allows the built-in LEAST_REQUEST LB policy to be configured via the LB policy
 // extension point. See the :ref:`load balancing architecture overview
 // <arch_overview_load_balancing_types>` for more information.
+// [#next-free-field: 6]
 message LeastRequest {
   // The number of random healthy hosts from which the host with the fewest active requests will
   // be chosen. Defaults to 2 so that we perform two-choice selection if the field is not set.
@@ -58,4 +59,9 @@ message LeastRequest {
 
   // Configuration for local zone aware load balancing or locality weighted load balancing.
   common.v3.LocalityLbConfig locality_lb_config = 4;
+
+  // Configuration for performing full scan on the list of hosts.
+  // If this configuration is set, when selecting the host a full scan on the list hosts will be
+  // used to select the one with least requests instead of using random choices.
+  google.protobuf.BoolValue enable_full_scan = 5;
 }