public Builder maxSpanCount(int maxSpanCount) {
      if (maxSpanCount <= 0) throw new IllegalArgumentException("maxSpanCount <= 0");
      this.maxSpanCount = maxSpanCount;
      return this;
    }
Eldest traces are removed to ensure spans in memory don't exceed this value
zipkin/src/main/java/zipkin2/storage/InMemoryStorage.java