Metrics: record compaction drop's and bloom filter effectiveness

Summary: Record BloomFliter hits and drop off reasons during compaction.

Test Plan: Unit tests work.

Reviewers: dhruba, heyongqiang

Reviewed By: dhruba

Differential Revision: https://reviews.facebook.net/D6591

table/table.cc

@@ -243,6 +243,7 @@ Status Table::InternalGet(const ReadOptions& options, const Slice& k,
         handle.DecodeFrom(&handle_value).ok() &&
         !filter->KeyMayMatch(handle.offset(), k)) {
       // Not found
+      RecordTick(rep_->options.statistics, BLOOM_FILTER_USEFUL);
     } else {
       bool didIO = false;
       Iterator* block_iter = BlockReader(this, options, iiter->value(),