Fix "Interval WAL" bytes to say GB instead of MB (#8350)

Summary:
Reference: https://github.com/facebook/rocksdb/issues/7201

Before fix:
`/tmp/rocksdb_test_file/LOG.old.1622492586055679:Interval WAL: 0 writes, 0 syncs, 0.00 writes per sync, written: 0.00 MB, 0.00 MB/s`

After fix:
`/tmp/rocksdb_test_file/LOG:Interval WAL: 0 writes, 0 syncs, 0.00 writes per sync, written: 0.00 GB, 0.00 MB/s`

Tests:
```
Computer:jobs running/jobs completed/%of started jobs/Average seconds to complete
ETA: 0s Left: 0 AVG: 0.05s  local:0/7720/100%/0.0s
rm -rf /dev/shm/rocksdb.CLRh
/usr/bin/python3 tools/check_all_python.py
No syntax errors in 34 .py files
/usr/bin/python3 tools/ldb_test.py
Running testCheckConsistency...
.Running testColumnFamilies...
.Running testCountDelimDump...
.Running testCountDelimIDump...
.Running testDumpLiveFiles...
.Running testDumpLoad...
Warning: 7 bad lines ignored.
.Running testGetProperty...
.Running testHexPutGet...
.Running testIDumpBasics...
.Running testIngestExternalSst...
.Running testInvalidCmdLines...
.Running testListColumnFamilies...
.Running testManifestDump...
.Running testMiscAdminTask...
Sequence,Count,ByteSize,Physical Offset,Key(s)
.Running testSSTDump...
.Running testSimpleStringPutGet...
.Running testStringBatchPut...
.Running testTtlPutGet...
.Running testWALDump...
.
----------------------------------------------------------------------
Ran 19 tests in 15.945s

OK
sh tools/rocksdb_dump_test.sh
make check-format
make[1]: Entering directory '/home/piydatta/Documents/rocksdb'
$DEBUG_LEVEL is 1
Makefile:176: Warning: Compiling in debug mode. Don't use the resulting binary in production
build_tools/format-diff.sh -c
Checking format of uncommitted changes...
```

Pull Request resolved: https://github.com/facebook/rocksdb/pull/8350

Reviewed By: jay-zhuang

Differential Revision: D28790567

Pulled By: zhichao-cao

fbshipit-source-id: dcb1e4c124361156435122f21f0a288335b2c8c8

db/internal_stats.cc

@@ -1239,7 +1239,7 @@ void InternalStats::DumpDBStats(std::string* value) {
   snprintf(
       buf, sizeof(buf),
       "Interval WAL: %s writes, %s syncs, "
-      "%.2f writes per sync, written: %.2f MB, %.2f MB/s\n",
+      "%.2f writes per sync, written: %.2f GB, %.2f MB/s\n",
       NumberToHumanString(interval_write_with_wal).c_str(),
       NumberToHumanString(interval_wal_synced).c_str(),
       interval_write_with_wal / static_cast<double>(interval_wal_synced + 1),