Print histogram count and sum in statistics string

Summary:
Previously it only printed percentiles, even though our histogram keeps track of count and sum (and more). There have been many times we want to know more than the percentiles. For example, we currently want sum of "rocksdb.compression.times.nanos" and sum of "rocksdb.decompression.times.nanos", which would allow us to know the relative cost of compression vs decompression.

This PR adds count and sum to the string printed by `StatisticsImpl::ToString`. This is a bit risky as there are definitely parsers assuming the old format. I will mention it in HISTORY.md and hope for the best...
Closes https://github.com/facebook/rocksdb/pull/3863

Differential Revision: D8038831

Pulled By: ajkr

fbshipit-source-id: 0465b72e4b0cbf18ef965f4efe402601d16d5b5c

HISTORY.md

 # Rocksdb Change Log
 ## Unreleased
+### Public API Change
+* For users of `Statistics` objects created via `CreateDBStatistics()`, the format of the string returned by its `ToString()` method has changed.
 
 ## 5.14.0 (5/16/2018)
 ### Public API Change

include/rocksdb/statistics.h

@@ -619,6 +619,8 @@ struct HistogramData {
   // zero-initialize new members since old Statistics::histogramData()
   // implementations won't write them.
   double max = 0.0;
+  uint64_t count = 0;
+  uint64_t sum = 0;
 };
 
 enum StatsLevel {

monitoring/histogram.cc

@@ -235,6 +235,8 @@ void HistogramStat::Data(HistogramData * const data) const {
   data->max = static_cast<double>(max());
   data->average = Average();
   data->standard_deviation = StandardDeviation();
+  data->count = num();
+  data->sum = sum();
 }
 
 void HistogramImpl::Clear() {

monitoring/statistics.cc

@@ -169,11 +169,17 @@ std::string StatisticsImpl::ToString() const {
       char buffer[kTmpStrBufferSize];
       HistogramData hData;
       getHistogramImplLocked(h.first)->Data(&hData);
-      snprintf(
+      // don't handle failures - buffer should always be big enough and arguments
+      // should be provided correctly
+      int ret = snprintf(
           buffer, kTmpStrBufferSize,
-          "%s statistics Percentiles :=> 50 : %f 95 : %f 99 : %f 100 : %f\n",
-          h.second.c_str(), hData.median, hData.percentile95,
-          hData.percentile99, hData.max);
+          "%s P50 : %f P95 : %f P99 : %f P100 : %f COUNT : %" PRIu64 " SUM : %"
+          PRIu64 "\n", h.second.c_str(), hData.median, hData.percentile95,
+          hData.percentile99, hData.max, hData.count, hData.sum);
+      if (ret < 0 || ret >= kTmpStrBufferSize) {
+        assert(false);
+        continue;
+      }
       res.append(buffer);
     }
   }