private boolean withinRateRange(int currentSample, int policySample) {
        return currentSample < policySample;
    }
On the basis of service's If the specific service's 'trace segment's slow threshold' is not null. The same as
'samplingRate', if the specific service's 'samplingRate' is not null. Otherwise,Using the default sampling
policy.
<p>
The priority of sampling policy: 'trace segment's slow threshold' > 'samplingRate',no matter the service's or
global. When 'duration' is over 'default trace segment's slow threshold' that should be sampled. Or when 'sample'
is with in [0,defaultSamplingRate) that also should be sampled.

@param samplingPolicy the sampling policy of the specific service
@param sample         sample rate of trace segment
@param duration       duration of trace segment
@return
oap-server/analyzer/agent-analyzer/src/main/java/org/apache/skywalking/oap/server/analyzer/provider/trace/TraceSamplingPolicyWatcher.java