feat(compaction): metric by frequence and #sst side, compaction doc (#…

…829)

* feat(compaction): metric by sst_count together with bytes

* feat(compaction): add compact frequence stat for each level

* doc(compaction): add comments for compaction

* refactor(metrics): use counter rather than gauge for inc-only metrics

rust/storage/src/hummock/compactor.rs

@@ -244,6 +244,7 @@ impl Compactor {
             // Among keys with same user key, only retain keys which satisfy `epoch` >= `watermark`,
             // and the latest key which satisfies `epoch` < `watermark`
             if epoch < watermark {
+                // Only retain latest key which satisfies epoch < watermark
                 skip_key = BytesMut::from(iter_key);
                 if matches!(iter.value(), HummockValue::Delete) && !has_user_key_overlap {
                     iter.next().await?;