feat(pageserver): add db rel count as feature flag property (#12632)

## Problem

As part of the reldirv2 rollout: LKB-197.


We will use number of db/rels as a criteria whether to rollout reldirv2
directly on the write path (simplest and easiest way of rollout). If the
number of rel/db is small then it shouldn't take too long time on the
write path.

## Summary of changes

* Compute db/rel count during basebackup.
* Also compute it during logical size computation.
* Collect maximum number of db/rel across all timelines in the feature
flag propeties.

---------

Signed-off-by: Alex Chi Z <chi@neon.tech>

test_runner/regress/test_feature_flag.py

@@ -50,11 +50,15 @@ def test_feature_flag(neon_env_builder: NeonEnvBuilder):
         )["result"]
     )
 
+    env.endpoints.create_start("main")  # trigger basebackup
     env.pageserver.http_client().force_refresh_feature_flag(env.initial_tenant)
 
     # Check if the properties exist
     result = env.pageserver.http_client().evaluate_feature_flag_multivariate(
         env.initial_tenant, "test-feature-flag"
     )
+
     assert "tenant_remote_size_mb" in result["properties"]
+    assert "tenant_db_count_max" in result["properties"]
+    assert "tenant_rel_count_max" in result["properties"]
     assert "tenant_id" in result["properties"]