enhance BraintrustSpanProcessor to support custom exporters

[colinbennettbrain]

This change makes BraintrustSpanProcessor support custom OpenTelemetry exporters in addition to the default Braintrust export path. Some users need to route traces through their existing OTEL pipeline (e.g. vendor-specific exporters, internal collectors, or multi-destination export) while still using Braintrust instrumentation. Previously, BraintrustSpanProcessor did not make that customization path explicit/easy.

• Added exporter=None to:
  • BraintrustSpanProcessor(...)
  • add_braintrust_span_processor(...)
• If exporter is provided, the processor uses it directly.
• In that case, api_key, parent, api_url, and headers are ignored.
• If exporter is not provided, behavior is unchanged (the default Braintrust OtelExporter is still created).

You can create your exporter as usual, then pass it via the new exporter argument.

from opentelemetry.exporter.otlp.proto.http.trace_exporter import OTLPSpanExporter
from braintrust.otel import BraintrustSpanProcessor

custom_exporter = OTLPSpanExporter(endpoint="http://localhost:4318/v1/traces")
processor = BraintrustSpanProcessor(exporter=custom_exporter)

Or with the helper:

from braintrust.otel import add_braintrust_span_processor
add_braintrust_span_processor(
    tracer_provider=provider,
    exporter=custom_exporter,
)

[Abhijeet Prasad (AbhiPrasad)]

Can we add more details to the PR description about why this change was needed, and what users have to now do to add custom exporters? This will be useful for the changelogs.