Add missing stopwatch and perf timer to DBImplReadOnly (#11521)

Summary:
The `rocksdb.db.get.micros` histogram is never updated if the DB is open in ReadOnly mode, as well as the `get_cpu_nanos` perf counter. An earlier PR (https://github.com/facebook/rocksdb/issues/4260) for some reason has only added the TODO line, not the accounting itself, so this one is intended to fix it, adding two lines to match [DBImplSecondary](https://github.com/facebook/rocksdb/blob/4dafa5b220c439b7f05eb598a2ab32b3152bedb5/db/db_impl/db_impl_secondary.cc#L366-L367).

Pull Request resolved: https://github.com/facebook/rocksdb/pull/11521

Reviewed By: cbi42

Differential Revision: D46577330

Pulled By: jowlyzhang

fbshipit-source-id: be147923e763af32bbc18fd6bdf3aff8ebf08aee

db/db_impl/db_impl_readonly.cc

@@ -46,7 +46,8 @@ Status DBImplReadOnly::Get(const ReadOptions& read_options,
         "`Env::IOActivity::kUnknown`");
   }
   assert(pinnable_val != nullptr);
-  // TODO: stopwatch DB_GET needed?, perf timer needed?
+  PERF_CPU_TIMER_GUARD(get_cpu_nanos, immutable_db_options_.clock);
+  StopWatch sw(immutable_db_options_.clock, stats_, DB_GET);
   PERF_TIMER_GUARD(get_snapshot_time);
 
   assert(column_family);