Clear context class loader from started threads (#7488)

Author: laurit

context/src/main/java/io/opentelemetry/context/StrictContextStorage.java

@@ -252,6 +252,7 @@ static PendingScopes create() {
       thread.setName("weak-ref-cleaner-strictcontextstorage");
       thread.setPriority(Thread.MIN_PRIORITY);
       thread.setDaemon(true);
+      thread.setContextClassLoader(null);
       thread.start();
     }
 

sdk-extensions/incubator/src/main/java/io/opentelemetry/sdk/extension/incubator/trace/LeakDetectingSpanProcessor.java

@@ -87,6 +87,7 @@ private static PendingSpans create(BiConsumer<String, Throwable> reporter) {
       thread.setName("weak-ref-cleaner-leakingspandetector");
       thread.setPriority(Thread.MIN_PRIORITY);
       thread.setDaemon(true);
+      thread.setContextClassLoader(null);
       thread.start();
       return pendingSpans;
     }

sdk/common/src/main/java/io/opentelemetry/sdk/internal/DaemonThreadFactory.java

@@ -45,6 +45,7 @@ public Thread newThread(Runnable runnable) {
     try {
       t.setDaemon(true);
       t.setName(namePrefix + "-" + counter.incrementAndGet());
+      t.setContextClassLoader(null);
     } catch (SecurityException e) {
       // Well, we tried.
     }