Tracing: added abstract tracing classes.

In order to be generic and to still stick to Java 6 requirement for
driver-core, abstract classes: TracingInfo and TracingInfoFactory are
introduced. Further implementations of tracing are expected to be placed apart from
driver-core and hence keep Java 8 (or higher) requirements away.

Author: wprzytula

driver-core/src/main/java/com/datastax/driver/core/tracing/TracingInfo.java

@@ -0,0 +1,35 @@
+/*
+ * Copyright (C) 2021 ScyllaDB
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.datastax.driver.core.tracing;
+
+/**
+ * An abstraction layer over instrumentation library API, corresponding to a logical span in the
+ * trace.
+ */
+public interface TracingInfo {
+
+  /**
+   * Starts a span corresponding to this {@link TracingInfo} object. Must be called exactly once,
+   * before any other method, at the beginning of the traced execution.
+   *
+   * @param name the name given to the span being created.
+   */
+  void setNameAndStartTime(String name);
+
+  /** Must be always called exactly once at the logical end of traced execution. */
+  void tracingFinished();
+}

driver-core/src/main/java/com/datastax/driver/core/tracing/TracingInfoFactory.java

@@ -0,0 +1,31 @@
+/*
+ * Copyright (C) 2021 ScyllaDB
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ * http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package com.datastax.driver.core.tracing;
+
+/** Factory of trace info objects. */
+public interface TracingInfoFactory {
+
+  /** Creates new trace info object, inheriting global context. */
+  TracingInfo buildTracingInfo();
+
+  /**
+   * Creates new trace info object, inheriting context from provided another trace info object.
+   *
+   * @param parent the trace info object to be set as the parent of newly created trace info object.
+   */
+  TracingInfo buildTracingInfo(TracingInfo parent);
+}