Add request_id in IODebugContext. (#8045)

Summary:
Add request_id in IODebugContext which will be populated by
    underlying FileSystem for IOTracing purposes. Update IOTracer to trace
    request_id in the tracing records. Provided API
    IODebugContext::SetRequestId which will set the request_id and enable
    tracing for request_id. The API hides the implementation and underlying
    file system needs to call this API directly.

Update DB::StartIOTrace API and remove redundant Env* from the
    argument as its not used and DB already has Env that is passed down to
    IOTracer.

Pull Request resolved: facebook/rocksdb#8045

Test Plan: Update unit test.

Differential Revision: D26899871

Pulled By: akankshamahajan15

fbshipit-source-id: 56adef52ee5af0fb3060b607c3af1ec01635fa2b
Signed-off-by: Changlong Chen <levisonchen@live.cn>

include/rocksdb/file_system.h

@@ -132,12 +132,36 @@ struct IODebugContext {
   // To be set by the FileSystem implementation
   std::string msg;
 
+  // To be set by the underlying FileSystem implementation.
+  std::string request_id;
+
+  // In order to log required information in IO tracing for different
+  // operations, Each bit in trace_data stores which corresponding info from
+  // IODebugContext will be added in the trace. Foreg, if trace_data = 1, it
+  // means bit at position 0 is set so TraceData::kRequestID (request_id) will
+  // be logged in the trace record.
+  //
+  enum TraceData : char {
+    // The value of each enum represents the bitwise position for
+    // that information in trace_data which will be used by IOTracer for
+    // tracing. Make sure to add them sequentially.
+    kRequestID = 0,
+  };
+  uint64_t trace_data = 0;
+
   IODebugContext() {}
 
   void AddCounter(std::string& name, uint64_t value) {
     counters.emplace(name, value);
   }
 
+  // Called by underlying file system to set request_id and log request_id in
+  // IOTracing.
+  void SetRequestId(const std::string& _request_id) {
+    request_id = _request_id;
+    trace_data |= (1 << TraceData::kRequestID);
+  }
+
   std::string ToString() {
     std::ostringstream ss;
     ss << file_path << ", ";