GH-36384: [Go] Schema: NumFields

[thorfour]

Rationale for this change

Previously if one wanted to iterate over the fields in the schema you would call the Fields() function and just iterate over the slice. However, due to this commit there is now an allocation and copy that happens when that's called. So to iterate over the fields without allocations one now must use the Field(i int) method; however that means a user must already know exactly how many fields are in the schema which isn't possible today.

This adds a simple NumFields() int method that returns the number of fields in a schema to allow a user to iterate over all the fields without having to copy them.

What changes are included in this PR?

Expose the number of fields in a schema for iteration over fields.

Single function added NumFields() int to schema

Are these changes tested?

N/A

Are there any user-facing changes?

Yes this is a new API

• Closes: [Go] Expose number of fields in schema #36384

[kou]

Could you open a new issue for non-MINOR change instead of using MINOR: title?
See also our MINOR definition: https://github.com/apache/arrow/blob/main/CONTRIBUTING.md#Minor-Fixes

[thorfour]

Could you open a new issue for non-MINOR change instead of using MINOR: title? See also our MINOR definition: https://github.com/apache/arrow/blob/main/CONTRIBUTING.md#Minor-Fixes

Ah my bad, missed that. Will open

[thorfour]

This addresses #36384

[github-actions]

⚠️ GitHub issue #36384 has been automatically assigned in GitHub to PR creator.

[conbench-apache-arrow]

Conbench analyzed the 6 benchmark runs on commit 575b0957.

There were 8 benchmark results indicating a performance regression:

• Commit Run on arm64-m6g-linux-compute at 2023-07-02 18:39:30Z

  • params=<STATIC_VECTOR(int)>, source=cpp-micro, suite=arrow-small-vector-benchmark

• Commit Run on arm64-t4g-linux-compute at 2023-07-02 18:46:34Z

  • params=, source=cpp-micro, suite=arrow-value-parsing-benchmark

• and 6 more (see the report linked below)

The full Conbench report has more details.