message: [storage] Track read latency from underlying store
  We should track read latency to spot any performance
  anomaly amongst different column families.
  Closes:
  Approved by: wqfish
diff: @@ -238,10 +238,14 @@ impl DB {
  pub fn get<S: Schema>(&self, schema_key: &S::Key) -> Result<Option<S::Value>> {
  let k = <S::Key as KeyCodec<S>>::encode_key(&schema_key)?;
  let cf_handle = self.get_cf_handle(S::COLUMN_FAMILY_NAME)?;
  + let time = std::time::Instant::now();
  - self.inner
  + let result = self
  + .inner
  .get_cf(cf_handle, &k)
  - .map_err(convert_rocksdb_err)?
  + .map_err(convert_rocksdb_err)?;
  + OP_COUNTER.observe_duration(&format!("db_get_{}", S::COLUMN_FAMILY_NAME), time.elapsed());
  + result
  .map(|raw_value| <S::Value as ValueCodec<S>>::decode_value(&raw_value))
  .transpose()
  }
  