Improved metric to avoid alerts

[ZanCorDX]

📝 Summary

Now big negative balance deltas will not be counted as subsidies.
This might miss some subsidies (we could have a withdrawal AND a subsidy) but it's not critical.

💡 Motivation and Context

While receiving the Nobel Prize, I got a BuilderNet alert "subsidies too high" and had to forfeit it.
I turns out the subsidies were doing cocaine.

✅ I have completed the following steps:

• Run make lint
• Run make test
• Added tests (if applicable)

[claude]

Claude finished @ZanCorDX's task in 1m 30s —— View job

---

Review of PR #891

• Gather context and read changed files
• Review code changes
• Post review feedback

---

Overall: The approach is reasonable — moving the filter into add_subsidy_value and consolidating inc_subsidized_blocks as a private helper keeps the logic centralized. A couple of items worth noting:

1. Silent filtering without trace (minor observability concern)

In the wallet_balance_watcher path, a filtered subsidy will still be logged via info!("Subsidy detected") at line 166, but the metric won't be recorded — this mismatch could be confusing during debugging. In the relay_submit path, a filtered subsidy is completely silent (no log, no metric).

Consider adding a warn! or debug! log inside add_subsidy_value when the filter triggers, so operators can see when subsidies are being dropped:

if value > MAX_POSSIBLE_SUBSIDY {
    tracing::debug!(?value, landed, "Filtering subsidy exceeding MAX_POSSIBLE_SUBSIDY");
    return;
}

2. Hardcoded threshold

0.1 ETH is a sensible heuristic for now, but if operational conditions change (e.g., higher subsidies become normal), this will silently suppress real data. A configuration parameter or at minimum a more visible constant (e.g., in a config module) would make this easier to adjust without a code change. Not blocking, just something to keep in mind.

No blocking issues found. The logic is correct, the constant computation is valid (10^17 wei = 0.1 ETH), and making inc_subsidized_blocks private with the count now tied to the filtered add_subsidy_value is a good consolidation.