add sampler header file

sdk/include/opentelemetry/sdk/trace/sampler.h

@@ -0,0 +1,56 @@
+#pragma once
+
+#include "opentelemetry/version.h"
+#include "opentelemetry/common/attribute_value.h"
+
+#include <memory>
+#include <map>
+#include <string>
+
+OPENTELEMETRY_BEGIN_NAMESPACE
+namespace sdk
+{
+namespace trace
+{
+class Sampler
+{
+public:
+  /**
+   * Initialize a new tracer.
+   * @param processor The span processor for this tracer. This must not be a
+   * nullptr.
+   */
+  virtual Decision shouldSample();
+
+  /**
+   * Returns the description of this {@code Sampler}. This may be displayed on debug pages or in the
+   * logs.
+   *
+   * <p>Example: "ProbabilitySampler{0.000100}"
+   *
+   * @return the description of this {@code Sampler}.
+   */
+  virtual std::string getDescription();
+};
+
+class Decision
+{
+public:
+  /**
+   * Return sampling decision whether span should be sampled or not.
+   *
+   * @return sampling decision.
+   */
+  virtual bool isSampled();
+
+  /**
+     * Return tags which will be attached to the span.
+     *
+     * @return attributes added to span. These attributes should be added to the span only for root
+     *     span or when sampling decision {@link #isSampled()} changes from false to true.
+     */
+  virtual std::map<std::string, common::AttributeValue> getAttributes();
+};
+}  // namespace trace
+}  // namespace sdk
+OPENTELEMETRY_END_NAMESPACE