Add USDT static tracepoints to pgstat_report_wait_start/end

Add wait__event__start and wait__event__end probes to the DTrace
provider definition and invoke them from the static inline functions
pgstat_report_wait_start() and pgstat_report_wait_end().

Because these functions are static inline, they get inlined at every
call site (~100 locations across 36 files), leaving no function symbol
for eBPF uprobes to attach to. USDT probes solve this: the compiler
emits a nop instruction at each inlined site with ELF .note.stapsdt
metadata, allowing eBPF tools to discover and attach to all call sites
with a single probe definition.

This enables full eBPF-based wait event tracing (e.g., with bpftrace)
without requiring hardware watchpoints or PostgreSQL source patches
beyond this change.

When built without --enable-dtrace, the probes compile to do {} while(0)
with zero overhead.

PoC: covers all wait events via the two central inline functions.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: Nik Samokhvalov

src/backend/utils/probes.d

@@ -91,4 +91,7 @@ provider postgresql {
 	probe wal__switch();
 	probe wal__buffer__write__dirty__start();
 	probe wal__buffer__write__dirty__done();
+
+	probe wait__event__start(unsigned int);
+	probe wait__event__end();
 };

src/include/utils/wait_event.h

@@ -12,6 +12,7 @@
 
 /* enums for wait events */
 #include "utils/wait_event_types.h"
+#include "utils/probes.h"
 
 extern const char *pgstat_get_wait_event(uint32 wait_event_info);
 extern const char *pgstat_get_wait_event_type(uint32 wait_event_info);
@@ -73,6 +74,8 @@ pgstat_report_wait_start(uint32 wait_event_info)
 	 * four-bytes, updates are atomic.
 	 */
 	*(volatile uint32 *) my_wait_event_info = wait_event_info;
+
+	TRACE_POSTGRESQL_WAIT_EVENT_START(wait_event_info);
 }
 
 /* ----------
@@ -86,6 +89,8 @@ pgstat_report_wait_end(void)
 {
 	/* see pgstat_report_wait_start() */
 	*(volatile uint32 *) my_wait_event_info = 0;
+
+	TRACE_POSTGRESQL_WAIT_EVENT_END();
 }