Default management.tracing.sampling.probability (0.10, unset in application.yml):

  $ for i in $(seq 101 115); do curl -s -X POST "http://localhost:8080/orders/$i" > /dev/null; done
  # 15 requests sent

  $ curl -s "http://localhost:9090/api/v1/query?query=orders_placed_total"
  # metric value: 15   -- all 15 counted, metrics are never subject to trace sampling

  $ curl -s "http://localhost:3200/api/search?limit=50"
  # trace count increased by 1  -- only ~1 of the 15 requests was sampled into a trace

Same 15 requests, same app, same collector. Every one produced a metric data point. Roughly
1 in 10 produced a trace, because trace sampling and metric recording are governed by two
completely different knobs, and only one of them defaults to "record everything."

--- with the fulltrace profile (management.tracing.sampling.probability=1.0) ---

  $ mvn -Dspring-boot.run.profiles=fulltrace spring-boot:run
  $ for i in $(seq 401 410); do curl -s -X POST "http://localhost:8080/orders/$i" > /dev/null; done
  # 10 requests sent

  $ curl -s "http://localhost:3200/api/search?limit=50"
  # post /orders traces increased by 10 -- 10 of 10, all of them present