Close tracing exporter during BatchTraceProcessor.shutdown()

[mshsheikh]

Summary

This PR ensures that BatchTraceProcessor.shutdown() closes the tracing exporter after the final batch has been flushed.

Problem

BackendSpanExporter creates a persistent httpx.Client for trace ingestion. Before this change, shutdown drained the queue but did not explicitly close the exporter, which left the HTTP client and connection pool open.

That can leave resources allocated longer than necessary, especially in:

• long-running services
• tests that repeatedly create and tear down tracing components
• worker processes or environments with frequent startup and shutdown cycles

Change

After the final synchronous flush, the shutdown path now looks up close() on the exporter and calls it when available:

close_fn = getattr(self._exporter, "close", None)
if callable(close_fn):
    try:
        close_fn()
    except Exception as exc:
        logger.warning(
            "[non-fatal] Tracing: exporter close failed: %s",
            exc,
        )

Behavior

• preserves the existing final flush behavior
• closes exporter resources after export completes
• keeps cleanup non-fatal
• remains compatible with custom exporters that do not implement close()

Notes

This change assumes the exporter is safe to close at processor shutdown time. If exporter ownership is later shared more broadly, that lifecycle may need to be made explicit.