feat(context): Add toString() for debugging purpose

Author: PerfectSlayer

components/context/src/main/java/datadog/context/SingletonContext.java

@@ -57,4 +57,9 @@ public int hashCode() {
     result = 31 * result + Objects.hashCode(this.value);
     return result;
   }
+
+  @Override
+  public String toString() {
+    return "SingletonContext{" + "index=" + this.index + ", value=" + this.value + '}';
+  }
 }

internal-api/src/main/java/datadog/trace/bootstrap/instrumentation/api/AgentTracer.java

@@ -606,6 +606,11 @@ public long getSpanId() {
     public AgentSpanContext context() {
       return this.spanContext;
     }
+
+    @Override
+    public String toString() {
+      return "ExtractedSpan{spanContext=" + this.spanContext + '}';
+    }
   }
 
   public static class NoopAgentSpan implements AgentSpan {