Add mempool tracepoints

Tracepoints for added, removed, replaced, and rejected transactions.

The removal reason is passed as string instead of a numeric value, since
the benefits of not having to maintain a redundant enum-string mapping
seem to outweigh the small cost of string generation.  The reject reason
is passed as string as well, although here the string does not have to
be generated but is readily available.

So far, tracepoint PRs typically included two demo scripts: a naive
bpftrace script to show raw tracepoint data and a bcc script for a more
refined view. However, as some of the ongoing changes to bpftrace
introduce a certain degree of unreliability (running some of the
existing bpftrace scripts was not possible with standard kernels and
bpftrace packages on latest stable Ubuntu, Debian, and NixOS), this PR
includes only a single bcc script that fuses the functionality of former
bpftrace and bcc scripts.

Author: virtu

contrib/tracing/README.md

@@ -286,3 +286,53 @@ Spent   a05880b8c77971ed0b9f73062c7c4cdb0ff3856ab14cbf8bc481ed571cd34b83:1
 Added   eb689865f7d957938978d6207918748f74e6aa074f47874724327089445b0960:0            5589696005 2094513 No
 Added   eb689865f7d957938978d6207918748f74e6aa074f47874724327089445b0960:1               1565556 2094513 No
 ```
+
+### mempool_monitor.py
+
+A BCC Python script producing mempool statistics and an event log. Based on the
+`mempool:added`, `mempool:removed`, `mempool:replaced`, and `mempool:rejected`
+tracepoints.
+
+Statistics include incidence and rate for each event type since the script was
+started (`total`) as well as during the last minute (`1 min`) and ten minutes
+(`10 min`). The event log shows mempool events in real time, each entry
+comprising a timestamp along with all event data available via the event's
+tracepoint.
+
+```console
+$ python3 contrib/tracing/mempool_monitor.py ./src/bitcoind
+```
+
+```
+ Mempool Monitor
+ Press CTRL-C to stop.
+
+ ┌─Event count───────────────────────┐  ┌─Event rate──────────────────────────┐
+ │ Event       total   1 min  10 min │  │ Event       total    1 min   10 min │
+ │ added      1425tx   201tx  1425tx │  │ added     4.7tx/s  3.4tx/s  4.7tx/s │
+ │ removed      35tx     4tx    35tx │  │ removed   0.1tx/s  0.1tx/s  0.1tx/s │
+ │ replaced     35tx     4tx    35tx │  │ replaced  0.1tx/s  0.1tx/s  0.1tx/s │
+ │ rejected      0tx     0tx     0tx │  │ rejected  0.0tx/s  0.0tx/s  0.0tx/s │
+ └───────────────────────────────────┘  └─────────────────────────────────────┘
+
+ ┌─Event log────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
+ │ 13:10:30Z added f9064ca5bfc87cdd191faa42bf697217cd920b2b94838c1f1192e4f06c4fd217 with feerate 8.92 sat/vB (981 sat, 110 vbytes)                                              │
+ │ 13:10:30Z added 53ffa3afbe57b1bfe423e1755ca2b52c5b6cb4aa91b8b7ee9cb694953f47f234 with feerate 5.00 sat/vB (550 sat, 110 vbytes)                                              │
+ │ 13:10:30Z added 4177df5e19465eb5e53c3f8b6830a293f57474921bc6c2ae89375e0986e1f0f9 with feerate 2.98 sat/vB (429 sat, 144 vbytes)                                              │
+ │ 13:10:30Z added 931a10d83f0a268768da75dc4b9e199f2f055f12979ae5491cc304ee10f890ea with feerate 3.55 sat/vB (500 sat, 141 vbytes)                                              │
+ │ 13:10:30Z added 4cf32b295723cc4ab73f2a2e51d4bb276c0042760a4c00a3eb9595b8ebb24721 with feerate 89.21 sat/vB (12668 sat, 142 vbytes)                                           │
+ │ 13:10:31Z replaced d1eecf9d662121322f4f31f0c2267a752d14bb3956e6016ba96e87f47890e1db with feerate 27.12 sat/vB received 23.3 seconds ago (7213 sat, 266 vbytes) with c412db908│
+ │ 9b7ed53f3e5e36d2819dd291278b59ccaabaeb17fd37c3d87fdcd57 with feerate 28.12 sat/vB (8351 sat, 297 vbytes)                                                                     │
+ │ 13:10:31Z added c412db9089b7ed53f3e5e36d2819dd291278b59ccaabaeb17fd37c3d87fdcd57 with feerate 28.12 sat/vB (8351 sat, 297 vbytes)                                            │
+ │ 13:10:31Z added b8388a5bdc421b11460bdf477d5a85a1a39c2784e7dd7bffabe688740424ea57 with feerate 25.21 sat/vB (3554 sat, 141 vbytes)                                            │
+ │ 13:10:31Z added 4ddb88bc90a122cd9eae8a664e73bdf5bebe75f3ef901241b4a251245854a98e with feerate 24.15 sat/vB (5072 sat, 210 vbytes)                                            │
+ │ 13:10:31Z added 19101e4161bca5271ad5d03e7747f2faec7793b274dc2f3c4cf516b7cef1aac3 with feerate 7.06 sat/vB (1080 sat, 153 vbytes)                                             │
+ │ 13:10:31Z removed d1eecf9d662121322f4f31f0c2267a752d14bb3956e6016ba96e87f47890e1db with feerate 27.12 sat/vB (7213 sat, 266 vbytes): replaced                                │
+ │ 13:10:31Z added 6c511c60d9b95b9eff81df6ecba5c86780f513fe62ce3ad6be2c5340d957025a with feerate 4.00 sat/vB (440 sat, 110 vbytes)                                              │
+ │ 13:10:31Z added 44d66f7f004bd52c46be4dff3067cab700e51c7866a84282bd8aab560a5bfb79 with feerate 3.15 sat/vB (448 sat, 142 vbytes)                                              │
+ │ 13:10:31Z added b17b7c9ec5acfbbf12f0eeef8e29826fad3105bb95eef7a47d2f1f22b4784643 with feerate 4.10 sat/vB (1348 sat, 329 vbytes)                                             │
+ │ 13:10:31Z added b7a4ad93554e57454e8a8049bfc0bd803fa962bd3f0a08926aa72e7cb23e2276 with feerate 1.01 sat/vB (205 sat, 202 vbytes)                                              │
+ │ 13:10:32Z added c78e87be86c828137a6e7e00a177c03b52202ce4c39029b99904c2a094b9da87 with feerate 11.00 sat/vB (1562 sat, 142 vbytes)                                            │
+ │                                                                                                                                                                              │
+ └──────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────────┘
+```