test: fix on demand activation test flakyness (#7180)

Warm-up (and the "tenant startup complete" metric update) happens in
a background tokio task. The tenant map is eagerly updated (can happen
before the task finishes).

The test assumed that if the tenant map was updated, then the metric
should reflect that. That's not the case, so we tweak the test to wait
for the metric.

Fixes #7158

test_runner/regress/test_timeline_size.py

@@ -20,6 +20,7 @@
     VanillaPostgres,
     wait_for_last_flush_lsn,
 )
+from fixtures.pageserver.http import PageserverHttpClient
 from fixtures.pageserver.utils import (
     assert_tenant_state,
     timeline_delete_wait_completed,
@@ -684,6 +685,13 @@ def assert_physical_size_invariants(sizes: TimelinePhysicalSizeValues):
     # XXX would be nice to assert layer file physical storage utilization here as well, but we can only do that for LocalFS
 
 
+def wait_for_tenant_startup_completions(client: PageserverHttpClient, count: int):
+    def condition():
+        assert client.get_metric_value("pageserver_tenant_startup_complete_total") == count
+
+    wait_until(5, 1.0, condition)
+
+
 def test_ondemand_activation(neon_env_builder: NeonEnvBuilder):
     """
     Tenants warmuping up opportunistically will wait for one another's logical size calculations to complete
@@ -767,10 +775,7 @@ def at_least_one_active():
     # That one that we successfully accessed is now Active
     expect_activated += 1
     assert pageserver_http.tenant_status(tenant_id=stuck_tenant_id)["state"]["slug"] == "Active"
-    assert (
-        pageserver_http.get_metric_value("pageserver_tenant_startup_complete_total")
-        == expect_activated - 1
-    )
+    wait_for_tenant_startup_completions(pageserver_http, count=expect_activated - 1)
 
     # The ones we didn't touch are still in Attaching
     assert (
@@ -790,10 +795,7 @@ def at_least_one_active():
         == n_tenants - expect_activated
     )
 
-    assert (
-        pageserver_http.get_metric_value("pageserver_tenant_startup_complete_total")
-        == expect_activated - 1
-    )
+    wait_for_tenant_startup_completions(pageserver_http, count=expect_activated - 1)
 
     # When we unblock logical size calculation, all tenants should proceed to active state via
     # the warmup route.
@@ -813,7 +815,7 @@ def all_active():
     assert (
         pageserver_http.get_metric_value("pageserver_tenant_startup_scheduled_total") == n_tenants
     )
-    assert pageserver_http.get_metric_value("pageserver_tenant_startup_complete_total") == n_tenants
+    wait_for_tenant_startup_completions(pageserver_http, count=n_tenants)
 
     # Check that tenant deletion/detach proactively wakes tenants: this is done separately to the main
     # body of the test because it will disrupt tenant counts