Add views for metrics about pageserver requests (#9008)

The metrics include a histogram of how long we need to wait for a
GetPage request, number of reconnects, and number of requests among
other things.

The metrics are not yet exported anywhere, but you can query them
manually.

Note: This does *not* bump the default version of the 'neon' extension. We
will do that later, as a separate PR. The reason is that this allows us to roll back
the compute image smoothly, if necessary. Once the image that includes the
new extension .so file with the new functions has been rolled out, and we're
confident that we don't need to roll back the image anymore, we can change
default extension version and actually start using the new functions and views.

This is what the view looks like:

```
postgres=# select * from neon_perf_counters ;
                metric                 | bucket_le |  value   
---------------------------------------+-----------+----------
 getpage_wait_seconds_count            |           |      300
 getpage_wait_seconds_sum              |           | 0.048506
 getpage_wait_seconds_bucket           |     2e-05 |        0
 getpage_wait_seconds_bucket           |     3e-05 |        0
 getpage_wait_seconds_bucket           |     6e-05 |       71
 getpage_wait_seconds_bucket           |    0.0001 |      124
 getpage_wait_seconds_bucket           |    0.0002 |      248
 getpage_wait_seconds_bucket           |    0.0003 |      279
 getpage_wait_seconds_bucket           |    0.0006 |      297
 getpage_wait_seconds_bucket           |     0.001 |      298
 getpage_wait_seconds_bucket           |     0.002 |      298
 getpage_wait_seconds_bucket           |     0.003 |      298
 getpage_wait_seconds_bucket           |     0.006 |      300
 getpage_wait_seconds_bucket           |      0.01 |      300
 getpage_wait_seconds_bucket           |      0.02 |      300
 getpage_wait_seconds_bucket           |      0.03 |      300
 getpage_wait_seconds_bucket           |      0.06 |      300
 getpage_wait_seconds_bucket           |       0.1 |      300
 getpage_wait_seconds_bucket           |       0.2 |      300
 getpage_wait_seconds_bucket           |       0.3 |      300
 getpage_wait_seconds_bucket           |       0.6 |      300
 getpage_wait_seconds_bucket           |         1 |      300
 getpage_wait_seconds_bucket           |         2 |      300
 getpage_wait_seconds_bucket           |         3 |      300
 getpage_wait_seconds_bucket           |         6 |      300
 getpage_wait_seconds_bucket           |        10 |      300
 getpage_wait_seconds_bucket           |        20 |      300
 getpage_wait_seconds_bucket           |        30 |      300
 getpage_wait_seconds_bucket           |        60 |      300
 getpage_wait_seconds_bucket           |       100 |      300
 getpage_wait_seconds_bucket           |  Infinity |      300
 getpage_prefetch_requests_total       |           |       69
 getpage_sync_requests_total           |           |      231
 getpage_prefetch_misses_total         |           |        0
 getpage_prefetch_discards_total       |           |        0
 pageserver_requests_sent_total        |           |      323
 pageserver_requests_disconnects_total |           |        0
 pageserver_send_flushes_total         |           |      323
 file_cache_hits_total                 |           |        0
(39 rows)
```

test_runner/regress/test_compute_metrics.py

@@ -0,0 +1,21 @@
+from fixtures.neon_fixtures import NeonEnv
+
+
+def test_compute_metrics(neon_simple_env: NeonEnv):
+    """
+    Test compute metrics, exposed in the neon_backend_perf_counters and
+    neon_perf_counters views
+    """
+    env = neon_simple_env
+    endpoint = env.endpoints.create_start("main")
+
+    conn = endpoint.connect()
+    cur = conn.cursor()
+
+    # We don't check that the values make sense, this is just a very
+    # basic check that the server doesn't crash or something like that.
+    #
+    # 1.5 is the minimum version to contain these views.
+    cur.execute("CREATE EXTENSION neon VERSION '1.5'")
+    cur.execute("SELECT * FROM neon_perf_counters")
+    cur.execute("SELECT * FROM neon_backend_perf_counters")

test_runner/regress/test_neon_extension.py

@@ -50,8 +50,8 @@ def test_neon_extension_compatibility(neon_env_builder: NeonEnvBuilder):
             # Ensure that the default version is also updated in the neon.control file
             assert cur.fetchone() == ("1.4",)
             cur.execute("SELECT * from neon.NEON_STAT_FILE_CACHE")
-            all_versions = ["1.4", "1.3", "1.2", "1.1", "1.0"]
-            current_version = "1.4"
+            all_versions = ["1.5", "1.4", "1.3", "1.2", "1.1", "1.0"]
+            current_version = "1.5"
             for idx, begin_version in enumerate(all_versions):
                 for target_version in all_versions[idx + 1 :]:
                     if current_version != begin_version: