[OTEL] Implement `force_flush` and `shutdown` on `SentrySpanProcessor`

[lcian]

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Problem Statement

Currently we just no-op in those, we might want to provide an actual implementation. For instance, if the SpanProcessor is shut down, we could finish the ongoing Sentry spans, otherwise those will be lost.

Solution Brainstorm

Check where those functions are called and provide a suitable implementation.