Set the last_record_gauge to the value which was persisted metadata (#3460)

## Describe your changes
Whenever a tenant is detached or the pageserver is restarted the
pageserver_last_record_lsn metric is dropped
This fix resurrects the value from the metadata whenever the tenant is
attached again
## Issue ticket number and link
[3571](https://github.com/neondatabase/cloud/issues/3571)
## Checklist before requesting a review
- [X] I have performed a self-review of my code.
- [ ] If it is a core feature, I have added thorough tests.
- [ ] Do we need to implement analytics? if so did you add the relevant
metrics to the dashboard?
- [ ] If this PR requires public announcement, mark it with
/release-notes label and add several sentences in this section.

pageserver/src/tenant/timeline.rs

@@ -945,6 +945,10 @@ impl Timeline {
             };
             result.repartition_threshold = result.get_checkpoint_distance() / 10;
             result
+                .metrics
+                .last_record_gauge
+                .set(disk_consistent_lsn.0 as i64);
+            result
         })
     }
 

test_runner/regress/test_tenant_detach.py

@@ -6,6 +6,7 @@ from threading import Thread
 import asyncpg
 import pytest
 from fixtures.log_helper import log
+from fixtures.metrics import parse_metrics
 from fixtures.neon_fixtures import (
     NeonEnv,
     NeonEnvBuilder,
@@ -59,11 +60,11 @@ def test_tenant_reattach(
     # create new nenant
     tenant_id, timeline_id = env.neon_cli.create_tenant()
 
-    pg = env.postgres.create_start("main", tenant_id=tenant_id)
-    with pg.cursor() as cur:
-        cur.execute("CREATE TABLE t(key int primary key, value text)")
-        cur.execute("INSERT INTO t SELECT generate_series(1,100000), 'payload'")
-        current_lsn = Lsn(query_scalar(cur, "SELECT pg_current_wal_flush_lsn()"))
+    with env.postgres.create_start("main", tenant_id=tenant_id) as pg:
+        with pg.cursor() as cur:
+            cur.execute("CREATE TABLE t(key int primary key, value text)")
+            cur.execute("INSERT INTO t SELECT generate_series(1,100000), 'payload'")
+            current_lsn = Lsn(query_scalar(cur, "SELECT pg_current_wal_flush_lsn()"))
 
     # Wait for the all data to be processed by the pageserver and uploaded in remote storage
     wait_for_last_record_lsn(pageserver_http, tenant_id, timeline_id, current_lsn)
@@ -78,15 +79,34 @@ def test_tenant_reattach(
         ".*failed to perform remote task UploadMetadata.*, will retry.*"
     )
 
+    ps_metrics = parse_metrics(pageserver_http.get_metrics(), "pageserver")
+    tenant_metric_filter = {
+        "tenant_id": str(tenant_id),
+        "timeline_id": str(timeline_id),
+    }
+    pageserver_last_record_lsn_before_detach = int(
+        ps_metrics.query_one("pageserver_last_record_lsn", filter=tenant_metric_filter).value
+    )
+
     pageserver_http.tenant_detach(tenant_id)
     pageserver_http.tenant_attach(tenant_id)
 
-    with pg.cursor() as cur:
-        assert query_scalar(cur, "SELECT count(*) FROM t") == 100000
+    time.sleep(1)  # for metrics propagation
 
-    # Check that we had to retry the downloads
-    assert env.pageserver.log_contains(".*list prefixes.*failed, will retry.*")
-    assert env.pageserver.log_contains(".*download.*failed, will retry.*")
+    ps_metrics = parse_metrics(pageserver_http.get_metrics(), "pageserver")
+    pageserver_last_record_lsn = int(
+        ps_metrics.query_one("pageserver_last_record_lsn", filter=tenant_metric_filter).value
+    )
+
+    assert pageserver_last_record_lsn_before_detach == pageserver_last_record_lsn
+
+    with env.postgres.create_start("main", tenant_id=tenant_id) as pg:
+        with pg.cursor() as cur:
+            assert query_scalar(cur, "SELECT count(*) FROM t") == 100000
+
+        # Check that we had to retry the downloads
+        assert env.pageserver.log_contains(".*list prefixes.*failed, will retry.*")
+        assert env.pageserver.log_contains(".*download.*failed, will retry.*")
 
 
 num_connections = 10