Core: Support metrics with nested field in AppendBenchmark and FileGenerationUtil

[dramaticlly]

Adding schema support for nested fields in AppendBenchmark and FileGenerationUtil. Previous it will only handle the primitive type but real production workload often involves complex type.

fast	numFiles	Score (ms/op)
true (FastAppend)	50,000	519.3 ± 676.1
true (FastAppend)	100,000	876.3 ± 3311.6
false (MergeAppend)	50,000	484.0 ± 247.3
false (MergeAppend)	100,000	739.2 ± 395.2

[uros-b]

We have struct / list / map here, but how about variant? With v3 adoption growing, a schema containing variant will throw rather than skip. Let's at least consider adding

@Override
public Void variant(Types.VariantType v) {
  return null;
}

[dramaticlly]

I think current behavior for leafFields will throw UnsupportedOperationException if running into a V3 table with variant type. To me, it seems reasonable as exception serve as a reminder to add variant support instead of silently missing metrics or bound. There's also change like #15629 to add dedicated benchmark for variant.

Prefer to keep the existing scope and revisit v3+ type later in follow up PR

[steveloughran]

there is a variand benchmark, we need reviews of it!

[uros-b]

Here we only use MetricsConfig.getDefault(), but the new key code path for a nested field is never exercised under non-default modes (none, counts, truncate).

[dramaticlly]

Good call, parametrized on all metrics mode for better coverage

[dramaticlly]

@huaxingao @nastra if you want to take a look as well :)