fix: Fix diskNormalized cost balancer strategy

[jtuglu1]

Description

We've seen issues with the cost segment balancing strategy not working correctly in production, where it leaves historicals in a tier in a bimodal distribution: ~50% in ~100% utilization and ~50% in 60% utilization. This is due to the cost-based balancer not having any visibility into disk util on each historical. The diskNormalized strategy is available, but is marked as not "production-ready," I believe due to the following reason:

The “diskNormalized” balancer works by extending from the “cost” balancer and overriding the computeCost function in the following way:

double cost = super.computeCost(proposalSegment, server, includeCurrentServer);
if (cost == Double.POSITIVE_INFINITY) {
  return cost;
}
int nSegments = 1;
if (server.getServer().getNumSegments() > 0) {
  nSegments = server.getServer().getNumSegments();
}
double normalizedCost = cost / nSegments;
double usageRatio = (double) server.getSizeUsed() / (double) server.getServer().getMaxSize();
return normalizedCost * usageRatio;

This logic is a bit strange. Consider the case where historicals are all homogeneous, so server.getServer().getMaxSize() can be treated as a constant. In that case, the returned value simplifies to:

normalizedCost * usageRatio
= normalizedCost * usedSize / maxSize
= normalizedCost * usedSize / CONST
= cost / numSegments * usedSize / CONST
= cost * (usedSize / numSegments) / CONST
= cost * avgSegmentSize / CONST

So, in practice, it’s just adjusting cost by a factor of avgSegmentSize. This could lead to imbalances that make themselves worse over time, especially if newly created segments tend to have smaller sizes than older ones (which is common with append-based ingestion methods).

This fixes the diskNormalized strategy just weight the CostBalancerStrategy with the utilization of the server, defaulting to a 5% lenience threshold (TODO: make this adjustable via config?) to avoid ping-ponging segments between 2 historicals due to utilization differences.

Release note

Fix diskNormalized cost balancer strategy

---

This PR has:

• been self-reviewed.
  • using the concurrency checklist (Remove this item if the PR doesn't have any relation to concurrency.)
• added documentation for new or modified features or behaviors.
• a release note entry in the PR description.
• added Javadocs for most classes and all non-trivial methods. Linked related entities via Javadoc links.
• added or updated version, license, or notice information in licenses.yaml
• added comments explaining the "why" and the intent of the code wherever would not be obvious for an unfamiliar reader.
• added unit tests or modified existing tests to cover new code paths, ensuring the threshold for code coverage is met.
• added integration tests.
• been tested in a test Druid cluster.

[maytasm]

nit: diskUtilThresholdTolerance is a weird name. It doesn't indicate that a segment is already on the server. Maybe like a ...stickiness... or ...movingCost... is better?