[RocksDB] Add perf_context.wal_write_time to track time spent on writing the recovery log.

Summary: as title

Test Plan: make check; ./perf_context_test

Reviewers: dhruba

Reviewed By: dhruba

CC: leveldb

Differential Revision: https://reviews.facebook.net/D13629

include/rocksdb/perf_context.h

@@ -22,14 +22,18 @@ struct PerfContext {
   void Reset(); // reset all performance counters to zero
 
   uint64_t user_key_comparison_count; // total number of user key comparisons
-  uint64_t block_cache_hit_count;
-  uint64_t block_read_count;
-  uint64_t block_read_byte;
-  uint64_t block_read_time;
-  uint64_t block_checksum_time;
-  uint64_t block_decompress_time;
+  uint64_t block_cache_hit_count;     // total number of block cache hits
+  uint64_t block_read_count;          // total number of block reads (with IO)
+  uint64_t block_read_byte;           // total number of bytes from block reads
+  uint64_t block_read_time;           // total time spent on block reads
+  uint64_t block_checksum_time;       // total time spent on block checksum
+  uint64_t block_decompress_time;     // total time spent on block decompression
+  // total number of internal keys skipped over during iteration (overwritten or
+  // deleted, to be more specific, hidden by a put or delete of the same key)
   uint64_t internal_key_skipped_count;
+  // total number of deletes skipped over during iteration
   uint64_t internal_delete_skipped_count;
+  uint64_t wal_write_time;            // total time spent on writing to WAL
 };
 
 extern __thread PerfContext perf_context;