头部采样（Head Sampling）在请求入口即决定是否记录，无法感知后续错误；**尾部采样（Tail Sampling）**在 Collector 侧聚合完整 Trace 后再决策，可做到“错误与慢请求必留、正常请求按比例丢弃”。
一、Collector 尾部采样配置
yaml
processors:
tail_sampling:
decision_wait: 10s
policies:
- name: errors-policy
type: status_code
status_code: { status_codes: [ERROR] }
- name: slow-policy
type: latency
latency: { threshold_ms: 500 }
- name: probabilistic-policy
type: probabilistic
probabilistic: { sampling_percentage: 5 }