Fix a thread leak in SamplingService when updated sampling policy in the runtime. (#487)

Author: hyhyf

CHANGES.md

@@ -22,6 +22,7 @@ Release Notes.
 * Fix thrift plugin generate duplicate traceid when `sendBase` error occurs
 * Support keep trace profiling when cross-thread.
 * Fix unexpected whitespace of the command catalogs in several Redis plugins.
+* Fix a thread leak in `SamplingService` when updated sampling policy in the runtime.
 
 #### Documentation
 * Update docs of Tracing APIs, reorganize the API docs into six parts.

apm-sniffer/apm-agent-core/src/main/java/org/apache/skywalking/apm/agent/core/sampling/SamplingService.java

@@ -51,13 +51,16 @@ public class SamplingService implements BootService {
     private volatile ScheduledFuture<?> scheduledFuture;
 
     private SamplingRateWatcher samplingRateWatcher;
+    private ScheduledExecutorService service;
 
     @Override
     public void prepare() {
     }
 
     @Override
     public void boot() {
+        service = Executors.newSingleThreadScheduledExecutor(
+                new DefaultNamedThreadFactory("SamplingService"));
         samplingRateWatcher = new SamplingRateWatcher("agent.sample_n_per_3_secs", this);
         ServiceManager.INSTANCE.findService(ConfigurationDiscoveryService.class)
                                .registerAgentConfigChangeWatcher(samplingRateWatcher);
@@ -118,8 +121,6 @@ public void handleSamplingRateChanged() {
             if (!on) {
                 on = true;
                 this.resetSamplingFactor();
-                ScheduledExecutorService service = Executors.newSingleThreadScheduledExecutor(
-                    new DefaultNamedThreadFactory("SamplingService"));
                 scheduledFuture = service.scheduleAtFixedRate(new RunnableWithExceptionProtection(
                     this::resetSamplingFactor, t -> LOGGER.error("unexpected exception.", t)), 0, 3, TimeUnit.SECONDS);
                 LOGGER.debug(