WIP: page_service: add basic testcase for merging

The steps in the test work in neon_local + psql
but for some reason they don't work in the test.

Asked compute team on Slack for help:
https://neondb.slack.com/archives/C04DGM6SMTM/p1731952688386789

test_runner/regress/test_pageserver_getpage_merge.py

@@ -0,0 +1,70 @@
+
+import psycopg2.extras
+from fixtures.neon_fixtures import NeonEnvBuilder
+from fixtures.log_helper import log
+
+def test_getpage_merge_smoke(neon_env_builder: NeonEnvBuilder):
+    """
+    Do a bunch of sequential scans and ensure that the pageserver does some merging.
+    """
+
+    def patch_config_toml(ps_cfg):
+        ps_cfg["server_side_batch_timeout"] = "100ms"
+
+    neon_env_builder.pageserver_config_override = patch_config_toml
+    env = neon_env_builder.init_start()
+
+    ps_http = env.pageserver.http_client()
+
+    endpoint = env.endpoints.create_start("main")
+
+    with endpoint.connect() as conn:
+        with conn.cursor() as cur:
+
+            # cur.execute("SET max_parallel_workers_per_gather=0") # disable parallel backends
+            cur.execute("SET effective_io_concurrency=100")
+            # cur.execute("SET neon.readahead_buffer_size=128")
+            # cur.execute("SET neon.flush_output_after=1")
+
+            cur.execute("CREATE EXTENSION IF NOT EXISTS neon;")
+            cur.execute("CREATE EXTENSION IF NOT EXISTS neon_test_utils;")
+
+            log.info("Filling the table")
+            cur.execute("CREATE TABLE t (data char(1000)) with (fillfactor=10)")
+            cur.execute("INSERT INTO t SELECT generate_series(1, 1024)")
+            # TODO: can we force postgres to doe sequential scans?
+
+            def get_metrics():
+                with conn.cursor(cursor_factory=psycopg2.extras.RealDictCursor) as cur:
+                    cur.execute("select value from neon_perf_counters where metric='getpage_wait_seconds_count';")
+                    compute = cur.fetchall()
+
+                    pageserver_metrics  = ps_http.get_metrics()
+                    pageserver = {
+                        "getpage_count": pageserver_metrics.query_one("pageserver_smgr_query_seconds_count", {"smgr_query_type": "get_page_at_lsn"}),
+                        "vectored_get_count": pageserver_metrics.query_one("pageserver_get_vectored_seconds_count", {"task_kind": "PageRequestHandler"}),
+                    }
+                    return {
+                        "pageserver": pageserver,
+                        "compute": compute
+                    }
+
+            log.info("Doing bunch of seqscans")
+
+            for i in range(4):
+                log.info("Seqscan %d", i)
+                if i == 1:
+                    # round zero for warming up all the metrics
+                    before = get_metrics()
+                cur.execute("select clear_buffer_cache()") # TODO: what about LFC? doesn't matter right now because LFC isn't enabled by default in tests
+                cur.execute("select sum(data::bigint) from t")
+                assert cur.fetchall()[0][0] == sum(range(1, 1024 + 1))
+
+            after = get_metrics()
+
+
+    import pdb; pdb.set_trace()
+    # TODO: assert that getpage counts roughly match between compute and ps
+    # TODO: assert that batching occurs by asserting that vectored get count is siginificantly less than getpage count
+
+