[DONTREVIEW] Add test for unused subset removal

[rgs1]

We are tracking down why our active subsets keep growing without
the unused ones being removed, e.g.:

$ curl -s localhost:9901/stats | grep subset | grep app
cluster.app.lb_subsets_active: 396
cluster.app.lb_subsets_created: 396
cluster.app.lb_subsets_fallback: 1171992
cluster.app.lb_subsets_removed: 0
cluster.app.lb_subsets_selected: 46201647

We have the following subset LB config:

    "lb_subset_config": {
      "fallback_policy": "DEFAULT_SUBSET",
      "default_subset": {
        "default": "true"
      },
      "subset_selectors": [
        {
          "keys": [
            "default"
          ]
        },
        {
          "keys": [
            "version"
          ]
        },
        {
          "keys": [
            "canary"
          ]
        }
      ]
    }

And we never have more than have more than 6 possible subsets at a given
time:

default/true
version/v1
version/v2
version/v3
version/v4
canary/true

Of course, versions keep changing but there are never more than 4 at the same
time. It looks like the old ones are not being removed.

One important note is that endpoints remain constant (e.g.: (ip, port)), but
their metadata changes (e.g.: the version).

Signed-off-by: Raul Gutierrez Segales rgs@pinterest.com

[rgs1]

Here's how the test fail (while I fix format issues, etc), TLDR: active subsets keep
growing and no subset is removed.

Test output:

test/common/upstream/subset_lb_test.cc:578: Failure
Expected equality of these values:
  2U
    Which is: 2
  stats_.lb_subsets_active_.value()
    Which is: 3
test/common/upstream/subset_lb_test.cc:580: Failure
Expected equality of these values:
  1U
    Which is: 1
  stats_.lb_subsets_removed_.value()
    Which is: 0
[  FAILED  ] UpdateOrderings/SubsetLoadBalancerTest.UpdateMetadata/0, where GetParam() = 0 (4 ms)
[ RUN      ] UpdateOrderings/SubsetLoadBalancerTest.UpdateMetadata/1
test/common/upstream/subset_lb_test.cc:578: Failure
Expected equality of these values:
  2U
    Which is: 2
  stats_.lb_subsets_active_.value()
    Which is: 3
test/common/upstream/subset_lb_test.cc:580: Failure
Expected equality of these values:
  1U
    Which is: 1
  stats_.lb_subsets_removed_.value()
    Which is: 0
[  FAILED  ] UpdateOrderings/SubsetLoadBalancerTest.UpdateMetadata/1, where GetParam() = 1 (3 ms)

[mattklein123]

Assigning over to @zuercher to help figure out what's going on here.

[zuercher]

Closed in favor of #3814