[golang] encoding OTAP Logs adds empty trace_id & span_id columns to record batch

[albertlockett]

import(
	"go.opentelemetry.io/collector/pdata/plog"
	"github.com/open-telemetry/otel-arrow/pkg/otel/arrow_record"
)

// ...

logs := plog.NewLogs()
resourceLogs := logs.ResourceLogs().AppendEmpty()
scopeLogs := resourceLogs.ScopeLogs().AppendEmpty()
_ = scopeLogs.LogRecords().AppendEmpty()

producer := arrow_record.NewProducer()
bar, err := producer.BatchArrowRecordsFromLogs(logs)

This produces a single record batch with the following content:

| resource | scope   | time_unix_nano       | observed_time_unix_nano | trace_id                         | span_id          |
+----------+---------+----------------------+-------------------------+----------------------------------+------------------+
| {id: 0}  | {id: 0} | 1970-01-01T00:00:00Z | 1970-01-01T00:00:00Z    | 00000000000000000000000000000000 | 0000000000000000 |
+----------+---------+----------------------+-------------------------+----------------------------------+------------------+

I'm thinking it was probably not intentional that we add the trace_id & span_id columns to the record batch, especially considering in the schema metadata we mark them as optional columns:

otel-arrow/pkg/otel/logs/arrow/logs.go

Lines 48 to 49 in ff5ba27

	{Name: constants.TraceId, Type: &arrow.FixedSizeBinaryType{ByteWidth: 16}, Metadata: schema.Metadata(schema.Optional, schema.Dictionary8)},
	{Name: constants.SpanId, Type: &arrow.FixedSizeBinaryType{ByteWidth: 8}, Metadata: schema.Metadata(schema.Optional, schema.Dictionary8)},