Merge bitcoin/bitcoin#26531: mempool: Add mempool tracepoints

4b7aec2951 Add mempool tracepoints (virtu)

Pull request description:

  This PR adds multiple mempool tracepoints.

  | tracepoint  | description |
  | ------------- | ------------- |
  | `mempool:added`  | Is called when a transaction enters the mempool  |
  | `mempool:removed`  | ... when a transaction is removed from the mempool |
  | `mempool:replaced`  | ... when a transaction is replaced in the mempool |
  | `mempool:rejected`  | ... when a transaction is rejected from entering the mempool |

  The tracepoints are further documented in `docs/tracing.md`. Usage is demonstrated in the example script `contrib/tracing/mempool_monitor.py`. Interface tests are provided in `test/functional/interface_usdt_mempool.py`.

  The rationale for passing the removal reason as a string instead of numerically is that 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 in this instance the string does not have to be generated but is readily available.

ACKs for top commit:
  0xB10C:
    ACK 4b7aec2951
  achow101:
    ACK 4b7aec2951

Tree-SHA512: 6deb3ba2d1a061292fb9b0f885f7a5c4d11b109b838102d8a8f4828cd68f5cd03fa3fc64adc6fdf54a08a1eaccce261b0aa90c2b8c33cd5fd3828c8f74978958

test/functional/test_runner.py

@@ -195,6 +195,7 @@ BASE_SCRIPTS = [
     'interface_http.py',
     'interface_rpc.py',
     'interface_usdt_coinselection.py',
+    'interface_usdt_mempool.py',
     'interface_usdt_net.py',
     'interface_usdt_utxocache.py',
     'interface_usdt_validation.py',