Disable stable session id propagation until after tracer is registered (#11448)

Disable stable session id propagation until after tracer is registered

This avoids a potential reentrant situation when tracer debug is enabled:
1. The full `Config` is logged during startup in its constructor
2. This includes any lazy config fields, such as the `hostname`
3. If the host is not available in the environment/host-files then
   `Config` calls the `hostname` command using `Runtime.exec(...)`
4. The `Runtime.exec(...)` call is intercepted by the process advice
5. The process advice calls `Config.get()` to get the stable session id
6. `Config.get()` returns `null` as the config is still being built

The resulting NPE is caught before it escapes to the application,
but it still results in log-spam that could confuse investigations.

Co-authored-by: devflow.devflow-routing-intake <devflow.devflow-routing-intake@kubernetes.us1.ddbuild.io>

Author: mcculls

dd-java-agent/instrumentation/java/java-lang/java-lang-1.8/src/main/java/datadog/trace/instrumentation/java/lang/ProcessImplStartAdvice.java

@@ -12,16 +12,17 @@ class ProcessImplStartAdvice {
   public static AgentSpan beforeStart(
       @Advice.Argument(0) final String[] command,
       @Advice.Argument(1) final Map<String, String> environment) {
+
+    if (!AgentTracer.isRegistered()) {
+      return null;
+    }
+
     String rootSessionId = Config.get().getRootSessionId();
     if (rootSessionId != null && environment != null) {
       environment.put("_DD_ROOT_JAVA_SESSION_ID", rootSessionId);
     }
 
-    if (!ProcessImplInstrumentationHelpers.ONLINE) {
-      return null;
-    }
-
-    if (command.length == 0 || !AgentTracer.isRegistered()) {
+    if (!ProcessImplInstrumentationHelpers.ONLINE || command.length == 0) {
       return null;
     }