message: build/trace: measure write blob and bootstrap elapse
  Measure the two point of time consuming thus to better analyze
diff: @@ -561,7 +561,10 @@ impl Builder {
  /// Dump bootstrap and blob file, return (Vec<blob_id>, blob_size)
  fn dump_to_file(&mut self) -> Result<(Vec<String>, usize)> {
  - let (blob_hash, blob_size, mut blob_readahead_size) = self.dump_blob()?;
  + let (blob_hash, blob_size, mut blob_readahead_size) = timing_tracer!(
  + { self.dump_blob() },
  + "write all nodes to blob including hashing"
  + )?;
  // Set blob hash as blob id if not specified.
  if self.blob_id.is_empty() {
  @@ -667,6 +670,8 @@ impl Builder {
  .store(&mut self.f_bootstrap)
  .context("failed to store blob table")?;
  + timing_tracer!(
  + {
  for node in &mut self.nodes {
  if self.source_type == SourceType::StargzIndex {
  debug!("[{}]\t{}", node.overlay, node);
  @@ -680,6 +685,12 @@ impl Builder {
  .context("failed to dump bootstrap")?;
  }
  + Ok(())
  + },
  + "write all nodes to bootstrap",
  + Result<()>
  + )?;
  +
  let blob_ids: Vec<String> = self
  .blob_table
  .entries
  