Add span discard mechanism that drops unwanted spans before they enter
the batch export queue, saving both network bandwidth and storage.
FilteringSpanProcessor is a custom SpanProcessor decorator that wraps
BatchSpanProcessor. SpanGuard::discard() sets a thread-local flag
(tl_discardCurrentSpan) before calling Span::End(). The OTel SDK calls
OnEnd() synchronously on the same thread, where the flag is checked and
cleared to drop the span.
New file: DiscardFlag.h — zero-dependency header for the thread-local
flag, avoiding transitive include bloat from Telemetry.h.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
