Export GetCompactionReasonString/GetFlushReasonString by moving them …

…into listener.h (#11778)

Summary:
Currently, rocksdb users would use the event listener to catch the compaction/flush event and log them if any. But now the reason is an integer type instead of a human-readable string, so we would like to convert them into a human-readable string.

Pull Request resolved: #11778

Reviewed By: jaykorean

Differential Revision: D49012934

Pulled By: ajkr

fbshipit-source-id: a4935b95d70c1be02aec65da7bf1c98a8cf8b933

include/rocksdb/listener.h

@@ -161,6 +161,8 @@ enum class CompactionReason : int {
   kNumOfReasons,
 };
 
+const char* GetCompactionReasonString(CompactionReason compaction_reason);
+
 enum class FlushReason : int {
   kOthers = 0x00,
   kGetLiveFiles = 0x01,
@@ -180,6 +182,8 @@ enum class FlushReason : int {
   kWalFull = 0xd,
 };
 
+const char* GetFlushReasonString(FlushReason flush_reason);
+
 // TODO: In the future, BackgroundErrorReason will only be used to indicate
 // why the BG Error is happening (e.g., flush, compaction). We may introduce
 // other data structure to indicate other essential information such as