Report search_path to make it possible to use it in pgbouncer track_extra_parameters (#8303)

## Problem

When pooled connections are used, session semantic its not preserved,
including GUC settings.
Many customers have particular problem with setting search_path.
But pgbouncer 1.20 has `track_extra_parameters` settings which allows to
track parameters included in startup package which are reported by
Postgres. Postgres has [an official list of parameters that it reports
to the
client](https://www.postgresql.org/docs/15/protocol-flow.html#PROTOCOL-ASYNC).
This PR makes Postgres also report `search_path` and so allows to
include it in `track_extra_parameters`.



## Summary of changes

Set GUC_REPORT flag  for `search_path`.

## Checklist before requesting a review

- [ ] 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.

## Checklist before merging

- [ ] Do not forget to reformat commit message to not include the above
checklist

---------

Co-authored-by: Konstantin Knizhnik <knizhnik@neon.tech>

pgxn/neon/neon.c

@@ -32,6 +32,7 @@
 #include "utils/builtins.h"
 #include "utils/pg_lsn.h"
 #include "utils/guc.h"
+#include "utils/guc_tables.h"
 #include "utils/wait_event.h"
 
 #include "extension_server.h"
@@ -584,6 +585,40 @@ RestoreRunningXactsFromClog(CheckPoint *checkpoint, TransactionId **xids, int *n
 	return false;
 }
 
+
+/*
+ * pgbouncer is able to track GUCs reported by Postgres.
+ * But most parameters cannot be tracked this way. The only parameters that can be tracked are ones
+ * that Postgres reports to the client. Unfortunately `search_path` is not reported by Postgres:
+ * https://www.postgresql.org/message-id/flat/CAGECzQQ6xFcgrg%2Be0p9mCumtK362TiA6vTiiZKoYbS8OXggwuQ%40mail.gmail.com#be4bfd7a9cf1f0633bdb2d1790a0a1be
+ * This code sets GUC_REPORT flag for `search_path`making it possible to include it in
+ * pgbouncer's `track_extra_parameters` list.
+ *
+ * This code is inspired by how the Citus extension does this, see
+ * https://github.com/citusdata/citus/blob/2a263fe69a707d16ef24378f7650742386b0968f/src/backend/distributed/shared_library_init.c#L2694
+ */
+static void
+ReportSearchPath(void)
+{
+#if PG_VERSION_NUM >= 160000
+	int nGucs = 0;
+	struct config_generic **gucs = get_guc_variables(&nGucs);
+#else
+	struct config_generic **gucs = get_guc_variables();
+	int nGucs = GetNumConfigOptions();
+#endif
+
+	for (int i = 0; i < nGucs; i++)
+	{
+		struct config_generic *guc = (struct config_generic *) gucs[i];
+
+		if (strcmp(guc->name, "search_path") == 0)
+		{
+			guc->flags |= GUC_REPORT;
+		}
+	}
+}
+
 void
 _PG_init(void)
 {
@@ -627,6 +662,8 @@ _PG_init(void)
 	 * extension was loaded will be removed.
 	 */
 	EmitWarningsOnPlaceholders("neon");
+
+	ReportSearchPath();
 }
 
 PG_FUNCTION_INFO_V1(pg_cluster_size);