tests: fix metrics check in test_s3_eviction (#8419)

## Problem

This test would occasionally fail its metric check. This could happen in
the rare case that the nodes had all been restarted before their most
recent eviction.

The metric check was added in
#8348

## Summary of changes

- Check metrics before each restart, accumulate into a bool that we
assert on at the end of the test

test_runner/regress/test_wal_acceptor.py

@@ -2242,6 +2242,8 @@ def test_s3_eviction(
 
     check_values = [0] * n_timelines
 
+    event_metrics_seen = False
+
     n_iters = 20
     for _ in range(n_iters):
         if log.isEnabledFor(logging.DEBUG):
@@ -2266,6 +2268,27 @@ def test_s3_eviction(
         # update remote_consistent_lsn on pageserver
         ps_client.timeline_checkpoint(env.initial_tenant, timelines[i], wait_until_uploaded=True)
 
+        # Do metrics check before restarts, since these will reset to zero across a restart
+        event_metrics_seen |= any(
+            sk.http_client().get_metric_value(
+                "safekeeper_eviction_events_started_total", {"kind": "evict"}
+            )
+            or 0 > 0
+            and sk.http_client().get_metric_value(
+                "safekeeper_eviction_events_completed_total", {"kind": "evict"}
+            )
+            or 0 > 0
+            and sk.http_client().get_metric_value(
+                "safekeeper_eviction_events_started_total", {"kind": "restore"}
+            )
+            or 0 > 0
+            and sk.http_client().get_metric_value(
+                "safekeeper_eviction_events_completed_total", {"kind": "restore"}
+            )
+            or 0 > 0
+            for sk in env.safekeepers
+        )
+
         # restarting random safekeepers
         for sk in env.safekeepers:
             if random.random() < restart_chance:
@@ -2280,22 +2303,4 @@ def test_s3_eviction(
         for sk in env.safekeepers
     )
 
-    assert any(
-        sk.http_client().get_metric_value(
-            "safekeeper_eviction_events_started_total", {"kind": "evict"}
-        )
-        or 0 > 0
-        and sk.http_client().get_metric_value(
-            "safekeeper_eviction_events_completed_total", {"kind": "evict"}
-        )
-        or 0 > 0
-        and sk.http_client().get_metric_value(
-            "safekeeper_eviction_events_started_total", {"kind": "restore"}
-        )
-        or 0 > 0
-        and sk.http_client().get_metric_value(
-            "safekeeper_eviction_events_completed_total", {"kind": "restore"}
-        )
-        or 0 > 0
-        for sk in env.safekeepers
-    )
+    assert event_metrics_seen