Add metric for number of offloaded timelines (#11976)

We want to keep track of the number of offloaded timelines. It's a
per-tenant shard metric because each shard makes offloading decisions on
its own.

test_runner/fixtures/metrics.py

@@ -184,6 +184,7 @@ PAGESERVER_PER_TENANT_METRICS: tuple[str, ...] = (
     "pageserver_evictions_with_low_residence_duration_total",
     "pageserver_aux_file_estimated_size",
     "pageserver_valid_lsn_lease_count",
+    "pageserver_tenant_offloaded_timelines",
     counter("pageserver_tenant_throttling_count_accounted_start"),
     counter("pageserver_tenant_throttling_count_accounted_finish"),
     counter("pageserver_tenant_throttling_wait_usecs_sum"),

test_runner/regress/test_timeline_archive.py

@@ -193,6 +193,11 @@ def test_timeline_offloading(neon_env_builder: NeonEnvBuilder, manual_offload: b
         "test_ancestor_branch_archive_branch1", tenant_id, "test_ancestor_branch_archive_parent"
     )
 
+    offloaded_count = ps_http.get_metric_value(
+        "pageserver_tenant_offloaded_timelines", {"tenant_id": f"{tenant_id}"}
+    )
+    assert offloaded_count == 0
+
     ps_http.timeline_archival_config(
         tenant_id,
         leaf_timeline_id,
@@ -244,6 +249,11 @@ def test_timeline_offloading(neon_env_builder: NeonEnvBuilder, manual_offload: b
     wait_until(leaf_offloaded)
     wait_until(parent_offloaded)
 
+    offloaded_count = ps_http.get_metric_value(
+        "pageserver_tenant_offloaded_timelines", {"tenant_id": f"{tenant_id}"}
+    )
+    assert offloaded_count == 2
+
     # Offloaded child timelines should still prevent deletion
     with pytest.raises(
         PageserverApiException,