Log pending compaction bytes in a couple places (facebook#12267)

Summary:
This PR adds estimated pending compaction bytes in two places:

- The "Level summary", which is printed to the info LOG after every flush or compaction
- The "rocksdb.cfstats" property, which is printed to the info LOG periodically according to `stats_dump_period_sec`

Pull Request resolved: facebook#12267

Test Plan:
Ran `./db_bench -benchmarks=filluniquerandom -stats_dump_period_sec=1 -statistics=true -write_buffer_size=524288` and looked at the LOG.

```
** Compaction Stats [default] **
...
Estimated pending compaction bytes: 12117691
...
2024/01/22-13:15:12.283563 1572872 (Original Log Time 2024/01/22-13:15:12.283540) [/db_impl/db_impl_compaction_flush.cc:371] [default] Level summary: files[10 1 0 0 0 0 0] max score 0.50, estimated pending compaction bytes 12359137
```

Reviewed By: cbi42

Differential Revision: D52973337

Pulled By: ajkr

fbshipit-source-id: c4e546bd9bdac387eebeeba303d04125212037b8

db/internal_stats.cc

@@ -2067,6 +2067,12 @@ void InternalStats::DumpCFStatsNoFileHistogram(bool is_periodic,
  interval_compact_bytes_read / kMB / std::max(interval_seconds_up, 0.001),
  interval_compact_micros / kMicrosInSec);
  value->append(buf);
+
+ snprintf(buf, sizeof(buf),
+ "Estimated pending compaction bytes: %" PRIu64 "\n",
+ vstorage->estimated_compaction_needed_bytes());
+ value->append(buf);
+
  if (is_periodic) {
  cf_stats_snapshot_.compact_bytes_write = compact_bytes_write;
  cf_stats_snapshot_.compact_bytes_read = compact_bytes_read;

db/version_set.cc

@@ -4546,8 +4546,10 @@ const char* VersionStorageInfo::LevelSummary(
  // overwrite the last space
  --len;
  }
- len += snprintf(scratch->buffer + len, sizeof(scratch->buffer) - len,
- "] max score %.2f", compaction_score_[0]);
+ len +=
+ snprintf(scratch->buffer + len, sizeof(scratch->buffer) - len,
+ "] max score %.2f, estimated pending compaction bytes %" PRIu64,
+ compaction_score_[0], estimated_compaction_needed_bytes_);
 
  if (!files_marked_for_compaction_.empty()) {
  snprintf(scratch->buffer + len, sizeof(scratch->buffer) - len,