Remove the hard-coded enum value in statistics.h

Summary:
I am planning to add more to statistics classes but found current way of using enum is very verbose and unnecessarily increase the
difficulity of adding new statistics.

In this diff I removed the code that explicitly specifies the value of each enum entry. This will help us easily add new statistic
items more conveniently without manually adding the value of other enum entries by one.

Test Plan: make; make check;

Reviewers: haobo, dhruba, xjin, emayanke, vamsi

CC: leveldb

Differential Revision: https://reviews.facebook.net/D13197

include/rocksdb/statistics.h

@@ -23,58 +23,60 @@ namespace leveldb {
  * And incrementing TICKER_ENUM_MAX.
  */
 enum Tickers {
-  BLOCK_CACHE_MISS = 0,
-  BLOCK_CACHE_HIT = 1,
-  BLOOM_FILTER_USEFUL = 2, // no. of times bloom filter has avoided file reads.
+  BLOCK_CACHE_MISS,
+  BLOCK_CACHE_HIT,
+  BLOOM_FILTER_USEFUL, // no. of times bloom filter has avoided file reads.
+
   /**
    * COMPACTION_KEY_DROP_* count the reasons for key drop during compaction
    * There are 3 reasons currently.
    */
-  COMPACTION_KEY_DROP_NEWER_ENTRY = 3, // key was written with a newer value.
-  COMPACTION_KEY_DROP_OBSOLETE = 4, // The key is obsolete.
-  COMPACTION_KEY_DROP_USER = 5, // user compaction function has dropped the key.
-  // Number of keys written to the database via the Put and Write call's
-  NUMBER_KEYS_WRITTEN = 6,
-  // Number of Keys read,
-  NUMBER_KEYS_READ = 7,
-  // Bytes written / read
-  BYTES_WRITTEN = 8,
-  BYTES_READ = 9,
-  NO_FILE_CLOSES = 10,
-  NO_FILE_OPENS = 11,
-  NO_FILE_ERRORS = 12,
-  // Time system had to wait to do LO-L1 compactions
-  STALL_L0_SLOWDOWN_MICROS = 13,
-  // Time system had to wait to move memtable to L1.
-  STALL_MEMTABLE_COMPACTION_MICROS = 14,
-  // write throttle because of too many files in L0
-  STALL_L0_NUM_FILES_MICROS = 15,
-  RATE_LIMIT_DELAY_MILLIS = 16,
+  COMPACTION_KEY_DROP_NEWER_ENTRY, // key was written with a newer value.
+  COMPACTION_KEY_DROP_OBSOLETE, // The key is obsolete.
+  COMPACTION_KEY_DROP_USER, // user compaction function has dropped the key.
 
-  NO_ITERATORS = 17, // number of iterators currently open
+  // Number of keys written to the database via the Put and Write call's
+  NUMBER_KEYS_WRITTEN,
+  // Number of Keys read,
+  NUMBER_KEYS_READ,
+  // Bytes written / read
+  BYTES_WRITTEN,
+  BYTES_READ,
+  NO_FILE_CLOSES,
+  NO_FILE_OPENS,
+  NO_FILE_ERRORS,
+  // Time system had to wait to do LO-L1 compactions
+  STALL_L0_SLOWDOWN_MICROS,
+  // Time system had to wait to move memtable to L1.
+  STALL_MEMTABLE_COMPACTION_MICROS,
+  // write throttle because of too many files in L0
+  STALL_L0_NUM_FILES_MICROS,
+  RATE_LIMIT_DELAY_MILLIS,
+
+  NO_ITERATORS, // number of iterators currently open
 
   // Number of MultiGet calls, keys read, and bytes read
-  NUMBER_MULTIGET_CALLS = 18,
-  NUMBER_MULTIGET_KEYS_READ = 19,
-  NUMBER_MULTIGET_BYTES_READ = 20,
+  NUMBER_MULTIGET_CALLS,
+  NUMBER_MULTIGET_KEYS_READ,
+  NUMBER_MULTIGET_BYTES_READ,
 
   // Number of deletes records that were not required to be
   // written to storage because key does not exist
-  NUMBER_FILTERED_DELETES = 21,
-  NUMBER_MERGE_FAILURES = 22,
-  SEQUENCE_NUMBER = 23,
+  NUMBER_FILTERED_DELETES,
+  NUMBER_MERGE_FAILURES,
+  SEQUENCE_NUMBER,
 
   // number of times bloom was checked before creating iterator on a
   // file, and the number of times the check was useful in avoiding
   // iterator creation (and thus likely IOPs).
-  BLOOM_FILTER_PREFIX_CHECKED = 24,
-  BLOOM_FILTER_PREFIX_USEFUL = 25,
+  BLOOM_FILTER_PREFIX_CHECKED,
+  BLOOM_FILTER_PREFIX_USEFUL,
 
   // Number of times we had to reseek inside an iteration to skip
   // over large number of keys with same userkey.
-  NUMBER_OF_RESEEKS_IN_ITERATION = 26,
+  NUMBER_OF_RESEEKS_IN_ITERATION,
 
-  TICKER_ENUM_MAX = 27
+  TICKER_ENUM_MAX
 };
 
 // The order of items listed in  Tickers should be the same as
@@ -117,27 +119,27 @@ const std::vector<std::pair<Tickers, std::string>> TickersNameMap = {
  * And increment HISTOGRAM_ENUM_MAX
  */
 enum Histograms {
-  DB_GET = 0,
-  DB_WRITE = 1,
-  COMPACTION_TIME = 2,
-  TABLE_SYNC_MICROS = 3,
-  COMPACTION_OUTFILE_SYNC_MICROS = 4,
-  WAL_FILE_SYNC_MICROS = 5,
-  MANIFEST_FILE_SYNC_MICROS = 6,
+  DB_GET,
+  DB_WRITE,
+  COMPACTION_TIME,
+  TABLE_SYNC_MICROS,
+  COMPACTION_OUTFILE_SYNC_MICROS,
+  WAL_FILE_SYNC_MICROS,
+  MANIFEST_FILE_SYNC_MICROS,
   // TIME SPENT IN IO DURING TABLE OPEN
-  TABLE_OPEN_IO_MICROS = 7,
-  DB_MULTIGET = 8,
-  READ_BLOCK_COMPACTION_MICROS = 9,
-  READ_BLOCK_GET_MICROS = 10,
-  WRITE_RAW_BLOCK_MICROS = 11,
+  TABLE_OPEN_IO_MICROS,
+  DB_MULTIGET,
+  READ_BLOCK_COMPACTION_MICROS,
+  READ_BLOCK_GET_MICROS,
+  WRITE_RAW_BLOCK_MICROS,
 
-  STALL_L0_SLOWDOWN_COUNT = 12,
-  STALL_MEMTABLE_COMPACTION_COUNT = 13,
-  STALL_L0_NUM_FILES_COUNT = 14,
-  HARD_RATE_LIMIT_DELAY_COUNT = 15,
-  SOFT_RATE_LIMIT_DELAY_COUNT = 16,
-  NUM_FILES_IN_SINGLE_COMPACTION = 17,
-  HISTOGRAM_ENUM_MAX = 18
+  STALL_L0_SLOWDOWN_COUNT,
+  STALL_MEMTABLE_COMPACTION_COUNT,
+  STALL_L0_NUM_FILES_COUNT,
+  HARD_RATE_LIMIT_DELAY_COUNT,
+  SOFT_RATE_LIMIT_DELAY_COUNT,
+  NUM_FILES_IN_SINGLE_COMPACTION,
+  HISTOGRAM_ENUM_MAX
 };
 
 const std::vector<std::pair<Histograms, std::string>> HistogramsNameMap = {