feat: only enalbe stress relief for the entire cluster together

[VinozzZ]

Which problem is this PR solving?

When making distributed trace decisions, load is more evenly balanced across nodes in the cluster. This balanced load means that, ideally, each node should experience a similar level of stress.

Because of this, the entire cluster should activate stress relief mode collectively, rather than allowing individual nodes to enter stress relief independently. If a single node goes into stress relief on its own, it will lead to missing span information across the cluster. This discrepancy would result in inconsistent trace decisions between the stressed node and the rest of the cluster, potentially impacting the reliability of trace sampling decisions.

Short description of the changes

• calculate stress relief activation based on only cluster_stress_level
• modify stress reasons to match with the configuration options
• add test for the new stress relief activation calculation