obs: review context usage in `(*EventsExporter).Start`

Consider the following code:

https://github.com/cockroachdb/cockroach/blob/d9ec7443a8bc749d12821d2e2c46aec870dd57ff/pkg/obs/event_exporter.go#L297-L332

When the context is canceled in `case <-ctx.Done()`, we don't return from `(*EventsExporter).Start()` because there's an attempt to "flush everything". The actual flushing to the `otelClient`, however, reuses the same `ctx` instance that we know is already canceled, so that will be a no-op (the error will be logged, but nothing will actually be exported).

We should either return on `ctx.Done()` (making it clear no export will take place), or use a different context in the call to `Export()` (care would need to be taken to avoid hanging since the context passed to `Start` is already canceled).

Jira issue: CRDB-36912