Small tweaks to logging to track the number of immutable memtables

Summary:
We see some write stalls because of number of unflushed memtables. With existing logging I couldn't figure out what's happening exactly. See internal task t11446054 for details if interested. This diff adds:
- logging of memtable creation at info level; I wanted it on multiple occasions for different reasons; also include number of immutable memtables,
- logging of number of remaining immutable memtables after a flush.

Test Plan: ran tests

Reviewers: sdong

Reviewed By: sdong

Subscribers: andrewkr, dhruba

Differential Revision: https://reviews.facebook.net/D58833

db/db_impl.cc

@@ -4921,9 +4921,10 @@ Status DBImpl::SwitchMemtable(ColumnFamilyData* cfd, WriteContext* context) {
       new_superversion = new SuperVersion();
     }
   }
-  Log(InfoLogLevel::DEBUG_LEVEL, db_options_.info_log,
-      "[%s] New memtable created with log file: #%" PRIu64 "\n",
-      cfd->GetName().c_str(), new_log_number);
+  Log(InfoLogLevel::INFO_LEVEL, db_options_.info_log,
+      "[%s] New memtable created with log file: #%" PRIu64
+      ". Immutable memtables: %d.\n",
+      cfd->GetName().c_str(), new_log_number, cfd->imm()->NumNotFlushed());
   mutex_.Lock();
   if (!s.ok()) {
     // how do we fail if we're not creating new log?

db/flush_job.cc

@@ -197,6 +197,7 @@ Status FlushJob::Run(FileMetaData* file_meta) {
     stream << vstorage->NumLevelFiles(level);
   }
   stream.EndArray();
+  stream << "immutable_memtables" << cfd_->imm()->NumNotFlushed();
 
   if (measure_io_stats_) {
     if (prev_perf_level != PerfLevel::kEnableTime) {