Added min/max/avg data block size output to sst_dump

Summary: Added min/max/avg data block size output to sst_dump. Output was added to the end of BlockBasedTable::DumpDataBlocks, so it appears after the data block details, at the very end of the dump file.

Test Plan:
```
./db_bench --benchmarks=fillrandom
./sst_dump --file=/tmp/rocksdbtest-xyz/dbbench/000007.sst --command=raw
tail -n 6 /tmp/rocksdbtest-xyz/dbbench/000007_dump.txt
```
```
Data Block Summary:
--------------------------------------
  # data blocks: 11336
  min data block size: 903
  max data block size: 2268
  avg data block size: 2245.363356
```

Reviewers: IslamAbdelRahman

Reviewed By: IslamAbdelRahman

Subscribers: andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D61815

table/block_based_table_reader.cc

@@ -8,6 +8,8 @@
 // found in the LICENSE file. See the AUTHORS file for names of contributors.
 #include "table/block_based_table_reader.h"
 
+#include <algorithm>
+#include <limits>
 #include <string>
 #include <utility>
 #include <vector>
@@ -1891,6 +1893,10 @@ Status BlockBasedTable::DumpDataBlocks(WritableFile* out_file) {
     return s;
   }
 
+  uint64_t datablock_size_min = std::numeric_limits<uint64_t>::max();
+  uint64_t datablock_size_max = 0;
+  uint64_t datablock_size_sum = 0;
+
   size_t block_id = 1;
   for (blockhandles_iter->SeekToFirst(); blockhandles_iter->Valid();
        block_id++, blockhandles_iter->Next()) {
@@ -1899,6 +1905,14 @@ Status BlockBasedTable::DumpDataBlocks(WritableFile* out_file) {
       break;
     }
 
+    Slice bh_val = blockhandles_iter->value();
+    BlockHandle bh;
+    bh.DecodeFrom(&bh_val);
+    uint64_t datablock_size = bh.size();
+    datablock_size_min = std::min(datablock_size_min, datablock_size);
+    datablock_size_max = std::max(datablock_size_max, datablock_size);
+    datablock_size_sum += datablock_size;
+
     out_file->Append("Data Block # ");
     out_file->Append(rocksdb::ToString(block_id));
     out_file->Append(" @ ");
@@ -1956,6 +1970,24 @@ Status BlockBasedTable::DumpDataBlocks(WritableFile* out_file) {
     }
     out_file->Append("\n");
   }
+
+  uint64_t num_datablocks = block_id - 1;
+  if (num_datablocks) {
+    double datablock_size_avg =
+        static_cast<double>(datablock_size_sum) / num_datablocks;
+    out_file->Append("Data Block Summary:\n");
+    out_file->Append("--------------------------------------");
+    out_file->Append("\n  # data blocks: ");
+    out_file->Append(rocksdb::ToString(num_datablocks));
+    out_file->Append("\n  min data block size: ");
+    out_file->Append(rocksdb::ToString(datablock_size_min));
+    out_file->Append("\n  max data block size: ");
+    out_file->Append(rocksdb::ToString(datablock_size_max));
+    out_file->Append("\n  avg data block size: ");
+    out_file->Append(rocksdb::ToString(datablock_size_avg));
+    out_file->Append("\n");
+  }
+
   return Status::OK();
 }