Reduce overhead of parquet metric creation

[xudong963]

I wonder if we should apply the same pattern to the other metrics (lazily initialize them) -- if you can get a few percent in this query maybe it would get us a few in the others

Originally posted by @alamb in #21637 (comment)

[alamb]

I think this will especially help the shorter clickbench queries where the overhead of opening the parquet files is substantial

[alamb]

Update here

• @xudong963 tried this in Lazy register parquet file metrics #22353 🏆

However, upon review I think it turns out we can't use lazy registration as that would change the semantics

• Lazy register parquet file metrics #22353 (review)

However, I do think we could reduce the overhead of creating metrics in general by avoiding the string allocations. I am copying my comments from #22353 (comment) here as I think they are relevant:

One challenge is that there are many many new strings with the file name. Maybe it is worth looking into if we could replace the owned string in Label with a Arc (as another PR)

datafusion/datafusion/physical-expr-common/src/metrics/mod.rs

Lines 526 to 527 in 3aefba7

	name: Cow<'static, str>,
	value: Cow<'static, str>,

It might be tricky however, as we would have to retain the special case for &'static str 🤔

Maybe we could make a new thing instead of Cow like

enum ArcCow {
  /// Static string
  Static(&'static str),
  /// Shared arc
  Shared(Arc<str>),
   /// Owned allocation
   Owned(String)
}

With some finagling (like impl Into<ArcCow> I suspect we could make it mostly backwards compatible with the existing APIs / callsites 🤔

[alamb]

Maybe

enum SharedStr {
  /// Static string
  Static(&'static str),
  /// Shared arc
  Shared(Arc<str>),
   /// Owned allocation
   Owned(String)
}

[alamb]

And instead of

        let row_groups_pruned_bloom_filter = MetricBuilder::new(metrics)
            .with_new_label("filename", filename.to_string())
            .with_type(MetricType::Summary)
            .pruning_metrics("row_groups_pruned_bloom_filter", partition);

        let limit_pruned_row_groups = MetricBuilder::new(metrics)
            .with_new_label("filename", filename.to_string())
            .with_type(MetricType::Summary)
            .pruning_metrics("limit_pruned_row_groups", partition);

It could look something like

        // Allocate a single copy of the filename
        let shared_filename = Arc::from(filename.as_str());

        let row_groups_pruned_bloom_filter = MetricBuilder::new(metrics)
            .with_new_label("filename", Arc::clone(&shared_filename))     // <---- Only copy Arc here, not new string
            .with_type(MetricType::Summary)
            .pruning_metrics("row_groups_pruned_bloom_filter", partition);  

        let limit_pruned_row_groups = MetricBuilder::new(metrics)
            .with_new_label("filename", Arc::clone(&shared_filename))     // <---- Only copy Arc here, not new string
            .with_type(MetricType::Summary)
            .pruning_metrics("limit_pruned_row_groups", partition);