contrib: tracing: Fix read of `pmsg_type` in p2p_monitor.py #32771

This fixes a bug in the contrib tracing script p2p_monitor.py. currently the script fails to read the msg_type of inbound and outbound messages, which is useful in the per-peer message view.

<details> <summary>Screenshot of p2p_monitor.py on master</summary>

</details>

<details> <summary>Screenshot of p2p_monitor.py on this branch</summary>

</details>

<!--e57a25ab6845829454e8d69fc972939a-->

The following sections might be updated with supplementary metadata relevant to reviewers and maintainers.

<!--006a51241073e994b41acfe9ec718e94-->

Code Coverage & Benchmarks

For details see: https://corecheck.dev/bitcoin/bitcoin/pulls/32771.

<!--021abf342d371248e50ceaed478a90ca-->

Reviews

See the guideline for information on the review process.

If your review is incorrectly listed, please react with 👎 to this comment and the bot will ignore it on the next update.

<!--5faf32d7da4f0f540f40219e4f7537a3-->

ut ACK 3473986fe10e2689fe36cc93e1e50013649ac14b

This PR fixes bug not displaying msg_type. bpf_probe_read_user_str(&msg.msg_type, sizeof(msg.msg_type), pmsg_type tries to copy pmsg_type into kernel space but pmsg_type was still NULL (unassigned). This PR changes bpf_usdt_readarg to copy argument 4 to pmsg_type fixing the bug.

Steps taken in this review:

• git blame (history of bug) ✅
• code review ✅

ACK 3473986fe10e2689fe36cc93e1e50013649ac14b

Changes look good to me, but I haven't tested them yet.

Introduced by me in https://github.com/bitcoin/bitcoin/commit/ec47ba349d0b3cb2d274593ca7b828ae70584e10

Concept ACK

It would be good to have the message type with the size of message in p2p_monitor.py

ACK 3473986fe10e2689fe36cc93e1e50013649ac14b

Testing

On master

<details> <summary>Screenshot</summary>

</details>

commit 3473986fe10e2689fe36cc93e1e50013649ac14b

<details> <summary>Screenshot</summary>

</details>

Backported to 29.x in #32589.