tracing: use bitcoind pid in bcc tracing examples

BCC needs the PID of a bitcoind process to attach to the tracepoints
(instead of the binary path used before) when the tracepoints have a
semaphore.

For reference, we already use the PID in our tracepoint interface
tests. See 220a5a2841.

contrib/tracing/mempool_monitor.py

@@ -114,8 +114,9 @@ int trace_replaced(struct pt_regs *ctx) {
 """
 
 
-def main(bitcoind_path):
-    bitcoind_with_usdts = USDT(path=str(bitcoind_path))
+def main(pid):
+    print(f"Hooking into bitcoind with pid {pid}")
+    bitcoind_with_usdts = USDT(pid=int(pid))
 
     # attaching the trace functions defined in the BPF program
     # to the tracepoints
@@ -365,8 +366,8 @@ class Dashboard:
 
 if __name__ == "__main__":
     if len(sys.argv) < 2:
-        print("USAGE: ", sys.argv[0], "path/to/bitcoind")
+        print("USAGE: ", sys.argv[0], "<pid of bitcoind>")
         exit(1)
 
-    path = sys.argv[1]
-    main(path)
+    pid = sys.argv[1]
+    main(pid)